Fix Slow Synology SMB and AFP Performance Issues

Synology

A Synology NAS connected to a fast network should not feel slow when opening files or transferring data. Yet users sometimes upgrade to 2.5GbE or 10GbE and still experience disappointing SMB performance, slow Finder browsing, or transfer rates barely better than their old network.

The NAS itself is not necessarily the problem. Synology file-sharing performance depends on the entire path between storage and client: disks, RAID configuration, DSM settings, SMB version, network adapters, switches, MTU, cabling, and Windows or macOS configuration. Before buying faster hardware, work through the bottlenecks systematically.

Start by Identifying the Actual Bottleneck

A slow file transfer can originate from several places:

NAS drives → storage pool → CPU → NAS NIC → switch → client NIC → operating system → application

Testing only Finder or Windows File Explorer makes it difficult to identify which component is responsible.

First determine whether the problem affects:

Large sequential transfers
Thousands of small files
Reads only
Writes only
One computer
Every computer
Wired Ethernet
Wi-Fi
SMB only

Large-file testing is particularly useful because it reduces the influence of file-count and metadata overhead. A single 20 GB file transferring slowly suggests a different problem from 50,000 tiny files transferring slowly.

Check Your SMB Protocol Settings

Modern Windows and macOS systems should generally use SMB2 or SMB3 rather than legacy SMB1.

Synology currently recommends modern SMB configurations and strongly advises against SMB1 because of its known security problems. DSM supports SMB2, SMB3, encryption, Large MTU, file leasing, directory leasing, and other performance-related SMB capabilities.

In DSM, check:

Control Panel → File Services → SMB → Advanced Settings

Review the minimum and maximum SMB protocol settings.

For a modern environment, SMB2 as the minimum and SMB3 as the maximum is a sensible starting point where all required clients support them. Synology’s current enterprise storage guidance similarly recommends SMB2 minimum and SMB3 maximum.

Avoid enabling old protocols merely to troubleshoot speed unless a genuinely old device requires them.

Understand SMB3 Multichannel

SMB3 Multichannel is one of the most useful Synology features for environments with multiple network interfaces. It allows SMB to establish multiple network connections between a client and NAS simultaneously.

The benefits can include:

Higher aggregate SMB throughput
Better utilization of multiple NICs
Network-path redundancy
Reduced CPU bottlenecks with RSS-capable adapters

Synology currently supports SMB3 Multichannel on compatible x86 NAS models running DSM 7.1.1 or later with SMB Service 4.15 or later. Supported clients include Windows 8/Server 2012 or later and macOS 11.3 or later.

SMB Multichannel Is Not the Same as Link Aggregation

This distinction causes considerable confusion. Suppose your Synology has two 1GbE interfaces. Creating a 2Gbps link aggregation bond does not necessarily allow one computer to transfer one file at 2Gbps.

Link aggregation is primarily valuable for distributing network capacity among multiple clients. SMB3 Multichannel can combine multiple network connections for a single SMB client, potentially increasing that client’s throughput. Synology specifically distinguishes the two this way: Multichannel can increase throughput for an individual SMB client, while Link Aggregation improves overall server bandwidth across clients without increasing the throughput of an individual client.

DSM also does not support using SMB3 Multichannel and Link Aggregation concurrently.

Verify That Multichannel Can Actually Help

Simply checking Enable SMB3 Multichannel does not guarantee higher speeds.

Synology requires appropriate networking on both ends. Systems need multiple adapters or suitable RSS-capable network adapters. Hardware elsewhere in the path must also be capable of delivering the expected performance.

For example, two 1GbE NAS connections are not very useful for accelerating one client if that computer has only one ordinary 1GbE interface.

Likewise, HDD performance or CPU limitations may prevent Multichannel from reaching the theoretical network maximum.

Always distinguish link speed from real storage throughput.

Check MTU Before Experimenting With Jumbo Frames

MTU settings are another frequent source of Synology performance problems.

Jumbo frames can reduce networking overhead in certain high-throughput environments, particularly on faster networks. But randomly setting the NAS to an MTU of 9000 does not automatically improve performance.

The complete path needs compatible configuration.

That may include:

NAS → switch → intermediate network equipment → client

A mismatch can cause poor performance or connectivity problems.

Synology’s April 2026 Windows SMB troubleshooting guidance specifically recommends checking the active interface’s MTU and ensuring that it matches the client. If problems continue, Synology recommends returning to the default MTU configuration.

For many environments, standard MTU is perfectly adequate.

Treat jumbo frames as an optimization to test not a requirement for fast SMB.

Review SMB Signing and Encryption

Security features can have a performance cost.

Synology’s current DSM specifications explicitly note that enabling SMB transport encryption or server signing may reduce SMB read/write performance.

That does not mean these protections should simply be disabled.

Business environments may require signing or encryption because of security policies and threat models.

Instead, recognize them as possible variables when diagnosing unexpected performance.

If performance drops dramatically after a policy change, investigate whether signing or encryption is involved before blaming the disks or network.

Security requirements should ultimately take priority over marginal benchmark gains.

Check Opportunistic Locking and SMB Leasing

DSM provides Opportunistic Locking to improve client-side caching and reduce small read/write overhead. It also supports SMB2 file leasing and SMB3 directory leasing. These capabilities can improve normal file-sharing workloads because clients can cache certain information instead of repeatedly communicating with the server. However, database-style applications may behave differently. Synology notes that disabling Opportunistic Locking can improve performance for database access to network-stored files. This is why there is no single “fastest SMB settings” configuration for every workload. File servers and database workloads have different access patterns.

Windows: Verify the Network Connection First

For Windows users, begin below the SMB layer. Check the negotiated Ethernet speed.

A computer with a 2.5GbE adapter can still negotiate at 1Gbps because of:

Cabling
Switch limitations
Driver problems
Adapter configuration
Faulty ports

Likewise, a 10GbE connection does not guarantee 10Gbps file transfers. The NAS storage must deliver data quickly enough to use that bandwidth. For multi-gigabit environments, keep NIC drivers current and verify that Windows recognizes the expected link speed before changing DSM settings.

Windows: Test Outside File Explorer

File Explorer is useful for everyday work but should not be your only diagnostic tool. Test raw network throughput independently from NAS storage performance where possible. Then test a large sequential file. If network testing shows several gigabits per second but SMB transfers remain dramatically slower, focus on SMB, storage, CPU utilization, signing, encryption, and client configuration.

If raw network throughput is already poor, DSM tuning probably will not fix the underlying issue.

macOS: Prefer SMB for Modern Environments

Synology continues to document AFP support, including Time Machine capabilities, but DSM also provides extensive modern SMB support for macOS clients.

For current mixed Windows/macOS environments, standardizing on SMB can simplify file-service management instead of maintaining parallel SMB and AFP access unless AFP is needed for a specific legacy workflow.

This also allows compatible Macs to take advantage of DSM’s SMB3 capabilities, including Multichannel.

macOS: Finder Slowness Is Not Always Transfer Slowness

A Mac can copy a large file quickly while Finder still feels slow browsing a folder containing thousands of files. These are different workloads. Finder may perform significant metadata and directory activity. Applications can also generate hidden files and extended attributes that add operations beyond simply reading the file contents.

When diagnosing macOS performance, therefore, compare:

Finder folder browsing
Large-file copying
Small-file copying
Application access
SMB throughput

Do not assume that slow directory browsing proves the physical network is slow.

Watch DSM Performance Settings During Troubleshooting

DSM includes SMB performance analysis capabilities that can help diagnose file-service behaviour. However, Synology warns that enabling performance analysis itself may decrease SMB performance. Use diagnostic features while troubleshooting rather than leaving every form of detailed analysis permanently enabled. The same principle applies to extensive transfer logging. Synology notes that increasing the number of monitored file-operation events can affect system performance.

Collect enough information to diagnose the problem without turning monitoring itself into another bottleneck.

Don’t Ignore the Storage Pool

Sometimes SMB is configured perfectly and the disks simply cannot go any faster.

Performance varies depending on:

HDD versus SSD
Number of drives
RAID/SHR configuration
Drive speed
Simultaneous workloads
Fragmentation and utilization
Encryption
Background operations

A NAS rebuilding an array, running backup jobs, indexing files, recording surveillance video, and serving several users simultaneously may deliver very different SMB performance from the same NAS sitting idle.

Monitor CPU, memory, disk, volume, and network activity while performing your test.

A Better Synology SMB Troubleshooting Order

Rather than changing ten DSM options at once, troubleshoot from the bottom upward:

Test storage performance and system utilization.
Verify Ethernet link speeds.
Test raw network throughput.
Check switch ports and cabling.
Confirm MTU consistency.
Verify SMB2/SMB3 configuration.
Test one large file over SMB.
Evaluate signing/encryption requirements.
Test SMB3 Multichannel where supported.
Compare Windows and macOS clients.
Investigate small-file and application-specific performance separately.

Changing one variable at a time makes it much easier to identify what actually improved performance.

When Faster Networking Is the Real Answer

Software tuning cannot overcome a physical 1GbE ceiling.

A single 1GbE connection provides roughly 125 MB/s of theoretical raw bandwidth before protocol overhead. If large-file transfers are already approaching the practical limit of that connection, changing SMB settings is unlikely to produce a dramatic improvement.

At that point, consider:

2.5GbE
5GbE
10GbE
SMB3 Multichannel
Faster switches
SSD storage
Additional drives
Higher-performance NAS hardware

The upgrade should target the bottleneck actually limiting the workload. Resolve SMB bottlenecks with a Synology Performance Review.

About Epis Technology

Epis Technology helps businesses diagnose and optimize Synology storage and network performance rather than relying on random DSM tweaks. Services include Synology deployment and troubleshooting, SMB configuration, Windows and macOS file-sharing optimization, 2.5GbE, 10GbE and 25GbE networking, storage and RAID planning, large storage solutions, business backups, virtualization, cybersecurity, and ongoing Synology support. Epis Technology can evaluate the complete path from NAS storage to client devices and identify whether SMB configuration, network infrastructure, storage performance, or client settings are limiting real-world file-transfer speeds.