How to Test Synology Network Speed and Find the Cause of Slow Transfers

What Slow Synology Network Speed Looks Like

A Synology NAS may appear slow when copying files, opening shared folders, uploading backups, or accessing applications. The transfer rate can change significantly between small files and large files. Large sequential files usually reveal network throughput, while many small files are affected by latency, file system operations, metadata processing, and client performance.

Internet download or upload speed is not the same as local NAS speed. A local transfer between a computer and Synology mainly depends on the Ethernet or Wi-Fi link, router or switch, NAS processing, storage devices, and file-sharing protocol. Remote access also depends on the ISP, fiber or cable broadband upload capacity, router configuration, and latency.

Common Cause: Wi-Fi Conditions Limit Throughput

Wi-Fi is a frequent reason for inconsistent Synology transfer speeds. Signal strength, channel congestion, distance, wall construction, interference, and the client device's wireless standard can reduce real throughput. A device connected over 2.4 GHz may be much slower than one connected over a clear 5 GHz or 6 GHz band, even when the router advertises a high link rate.

To judge this cause, connect the computer to the router or switch with Ethernet and repeat the same transfer. If the wired result is substantially faster and more stable, the NAS may be working normally and the wireless path is the limiting factor.

Common Cause: Ethernet Link Negotiation Is Too Slow

A Synology port, switch port, router port, or cable can negotiate at 100 Mbps instead of 1 Gbps or faster. A damaged cable, an older switch, a loose connection, or incompatible hardware can cause this downgrade. The advertised network speed is only a link rate; actual file transfer speed is lower because of protocol overhead and storage processing.

Check the negotiated speed in DSM and on the connected computer. Use a known-good Cat5e or better cable for gigabit connections, inspect the switch and router port specifications, and replace one cable or port at a time. A stable 1 Gbps link should generally provide much higher local throughput than a 100 Mbps link when the disks and client can keep up.

Common Cause: The Router or Switch Is the Bottleneck

Local traffic can be limited by an older router, an unmanaged switch with lower-speed ports, power-saving behavior, or a topology that sends traffic through unnecessary devices. Some networks also use a mesh Wi-Fi system whose wireless backhaul reduces the bandwidth available to the NAS and client.

Identify the path between the computer and Synology. For a useful comparison, place both devices on the same gigabit or multi-gigabit switch, then repeat the test. If performance improves, inspect the router, mesh node, switch uplink, and VLAN or bridge configuration rather than changing DSM settings first.

Common Cause: Disk Performance or NAS Resource Usage

Storage can limit network transfers when the NAS is handling disk-intensive tasks. RAID calculations, drive scrubbing, indexing, thumbnail generation, antivirus scanning, backups, snapshots, database activity, and a nearly full volume can reduce available performance. Conventional hard drives also perform poorly with many simultaneous small-file operations.

During a test, monitor CPU, memory, disk utilization, volume activity, and network utilization in DSM Resource Monitor. If disk usage or CPU usage reaches a sustained high level while network utilization remains below the expected link capacity, the storage or NAS workload is likely the bottleneck.

Common Cause: File Size and Transfer Protocol Behavior

Small files transfer more slowly than large files because each file requires directory operations, metadata updates, permission checks, and protocol requests. SMB settings, encryption, signing, compatibility options, and client operating system behavior can also change results. A test based on a folder containing thousands of small documents does not measure the same thing as a test using one large video file.

Compare a single large file with a folder containing many small files. Use a current SMB client, confirm that the shared folder uses the expected protocol, and avoid judging network capacity from a single application workflow. Do not disable security features blindly; first determine whether they are necessary for the network and whether CPU usage rises during transfers.

Common Cause: Remote Access Depends on the ISP and Upload Path

When the test is performed outside the home or office, the NAS's outbound bandwidth is usually more important than its local LAN speed. Many fiber, cable broadband, and other ISP plans provide different download and upload capacities. Carrier-grade NAT, a restrictive modem or router, VPN overhead, relay services, and high latency can further reduce remote performance.

Run a normal broadband test from the same location and compare its upload result with the remote NAS transfer. Test at different times to identify congestion. A remote transfer cannot exceed the weakest upstream link, and a fast local network cannot compensate for limited ISP upload capacity or unstable latency.

How to Test Synology Network Speed

1. Test the wired link. Connect the computer and Synology to the same capable switch with Ethernet and record the negotiated link rates.
2. Use a large test file. Copy a single large file in both directions and record the sustained rate after the transfer stabilizes.
3. Compare small files. Transfer a representative folder separately to expose latency and metadata overhead.
4. Monitor DSM resources. Check CPU, memory, disk, volume, and network activity during each test.
5. Compare network paths. Test direct wired access, Wi-Fi access, and remote access separately so their results are not mixed.
6. Use a throughput tool when needed. An iperf3 test between compatible endpoints can measure network capacity without disk activity, helping separate network limits from storage limits.

How to Interpret the Results

If iperf3 or another network-only test is fast but file copying is slow, investigate disks, CPU load, SMB behavior, encryption, and the client. If both network-only and file tests are slow, inspect link negotiation, cables, switches, router ports, Wi-Fi conditions, and VLAN paths.

If large files are fast but small files are slow, the result may be normal for the workload. If local transfers are fast but remote transfers are slow, focus on ISP upload capacity, VPN or relay overhead, router processing, and latency. Repeat each test more than once and compare sustained performance rather than a short peak value.

Ways to Improve Synology Network Performance

• Use wired Ethernet for the NAS and for performance-sensitive clients.
• Replace cables or switch ports that negotiate below the expected speed.
• Use a gigabit or multi-gigabit switch when all relevant devices support it.
• Move Wi-Fi clients closer to the router, select a less congested band, and improve mesh backhaul where applicable.
• Pause indexing, scrubbing, large backups, and other scheduled workloads while diagnosing performance.
• Keep DSM, network drivers, router firmware, and client operating systems current.
• Test large and small files separately and choose SMB settings that match the security and compatibility requirements.
• For remote access, verify upload capacity, reduce unnecessary VPN overhead, and measure latency at the time of the transfer.

When the Speed Is Still Low

Document the link rate, test file size, transfer direction, client device, network path, DSM resource usage, and time of day. This evidence helps distinguish a physical network problem from a storage or software issue. If the NAS remains slow on a direct wired test while resources are normal, test another client, another shared folder, and another cable before considering hardware failure or contacting Synology support.