Synology NAS Speed Test: Common Causes of Slow Performance

What a Synology NAS Speed Test Actually Measures

A Synology NAS speed test usually measures file transfer performance between a client device and the NAS. It does not necessarily measure your ISP download speed. A transfer between a computer and a NAS on the same local network mainly depends on the router, Ethernet or Wi-Fi link, network protocol, storage system, and NAS workload.

Test download and upload directions separately. In this context, download means copying data from the NAS to the client, while upload means copying data from the client to the NAS. Internet-based tests measure the connection between your router and the ISP, so they cannot by themselves confirm local NAS performance.

Common Symptoms of Slow Synology NAS Performance

• Large files transfer much more slowly than expected.
• Small files take a long time even when the network link appears fast.
• Performance changes significantly between Wi-Fi and Ethernet.
• Download speed is acceptable, but upload speed is unusually low, or the reverse.
• Transfers slow down when media indexing, backups, or surveillance recording is active.
• Remote access is slower than local access because of upload limits, latency, or relay connections.

Cause 1: The Client or NAS Is Using a Slow Network Link

A gigabit NAS cannot deliver gigabit-class transfers if the NAS, client, switch, or router has negotiated a 100 Mbps link. Damaged cables, older switches, loose connectors, and incompatible adapters can force a lower link speed. Check the negotiated Ethernet speed in the operating system and in DSM before interpreting the result.

For a useful baseline, connect the client and NAS directly to the router or a gigabit switch with tested Cat5e or better Ethernet cables. A local wired test removes most radio and roaming variables. If the transfer improves, the original bottleneck is in the network path rather than the NAS storage.

Cause 2: Wi-Fi Adds Interference and Shared Bandwidth

Wi-Fi performance varies with distance, walls, channel congestion, signal strength, and the number of active devices. A modern Wi-Fi connection may show a high theoretical link rate while delivering much less real-world throughput. The client may also switch between bands or access points during a transfer.

Repeat the Synology NAS speed test over Ethernet. Then compare a 5 GHz or 6 GHz connection with 2.4 GHz, test near the router, and pause other high-bandwidth activity. If wired performance is normal but Wi-Fi is slow, improve access point placement, select a less congested channel, and use a wired connection for large backups or media libraries.

Cause 3: Router, Switch, or Cable Configuration Limits Throughput

Network equipment can introduce a bottleneck even when both endpoints support gigabit Ethernet. A managed switch may have a speed or duplex setting that is incorrect, while an older router may contain 100 Mbps ports. Powerline adapters, mesh backhaul links, and inexpensive wireless bridges can also reduce throughput or add latency.

Inspect every segment between the client and NAS, including wall outlets and intermediate switches. Confirm that ports negotiate at the expected speed and that there are no excessive errors. Replace one component at a time and rerun the same large-file test so the limiting segment can be identified.

Cause 4: Storage Disks Cannot Sustain the Requested Transfer

NAS speed is affected by disk type, RAID layout, free capacity, fragmentation, disk health, and the size of the files being transferred. A single large file is usually faster than thousands of small files because small files create more metadata and seek operations. RAID calculations can also reduce write performance, especially during parity updates.

Check Storage Manager for disk warnings, volume utilization, RAID status, and ongoing data scrubbing or repair tasks. Test with a large file and compare read and write directions. Avoid treating a temporary slowdown during RAID repair or scrubbing as the normal capacity of the NAS.

Cause 5: NAS Background Services Consume CPU or Disk Resources

Media indexing, thumbnail generation, antivirus scanning, cloud synchronization, download tasks, virtual machines, containers, and surveillance recording can compete with file transfers. The effect is stronger on entry-level models with limited CPU, memory, or disk I/O capacity.

Open Resource Monitor in DSM while running the test. Watch CPU, memory, volume utilization, disk latency, and network usage. Schedule indexing, backups, and scrubbing outside peak transfer periods. Disable only services that are unnecessary for the test, and restore required services after the baseline measurement.

Cause 6: SMB Settings and File Characteristics Affect Results

SMB is commonly used for Windows and macOS file sharing, but its performance depends on protocol version, encryption, signing, permissions, antivirus inspection, and client behavior. SMB encryption and signing can add CPU overhead. Small files, nested folders, and frequent metadata operations can make a normal network look slow.

Use a large test file for baseline throughput, then test a representative folder separately. Keep SMB2 or SMB3 enabled, avoid changing advanced settings without a measured reason, and check whether encryption or signing is required by your security policy. Compare a new shared folder with the production folder to detect permission or indexing effects.

How to Run a Reliable Synology NAS Speed Test

1. Update DSM and the client operating system, then record the NAS model, disk layout, and network adapter speeds.
2. Connect the client and NAS to the same router or gigabit switch using Ethernet.
3. Pause unrelated transfers and background jobs for the first baseline test.
4. Copy one large file in both directions and record the sustained result after the transfer stabilizes.
5. Test a folder containing many small files to measure metadata-heavy performance.
6. Repeat the test over Wi-Fi and compare it with the wired baseline.
7. Monitor DSM Resource Monitor and note CPU, memory, disk, and network utilization.
8. Run an Internet speed test separately to compare local network performance with the ISP connection.

Practical Optimization Steps

• Use gigabit or faster Ethernet for the NAS and high-volume clients.
• Replace questionable cables and remove unnecessary powerline or wireless bridge segments.
• Use a clear 5 GHz or 6 GHz Wi-Fi channel when wired access is unavailable.
• Keep adequate free space on the volume and monitor disk health.
• Schedule backups, indexing, scrubbing, and other intensive jobs during low-use periods.
• Use large-file tests for throughput and small-file tests for workload realism.
• Review SMB security and encryption settings without weakening required protections.
• For remote access, check the NAS-side upload speed, router configuration, latency, and whether traffic is passing through a relay.

When the Result Is Normal

A gigabit network does not usually produce a full 1 Gbps of file transfer payload because protocol overhead, disk behavior, and system processing consume part of the link capacity. Results also vary by file size and workload. The useful question is whether performance is consistent with the slowest component and whether the result improves when that component is isolated.

If a wired large-file test is stable, the network link is correctly negotiated, and DSM shows no disk or resource problems, the observed speed may be normal for the NAS model and storage configuration. Compare results with the model's hardware limits and the specific workload instead of relying only on an Internet speed test.

For additional network diagnostics, use Speedtest.im to measure the ISP connection separately from local NAS transfers.