Quick Answer+
Quick Answer: Slow Synology transfer speeds are usually caused by: (1) network bottlenecks (bad cables, switch limitations), (2) SMB settings misconfiguration, (3) drive issues (failing disk, SMR drives), or (4) resource contention (background tasks). Start by testing with iperf to isolate network vs NAS issues, then check DSM settings and drive health.
Few things are more frustrating than a NAS that transfers files at a fraction of expected speeds. Whether you're seeing 20 MB/s instead of 100+ MB/s, or your once-fast NAS has slowed to a crawl, this guide covers systematic troubleshooting to identify and fix the problem.
Expected Transfer Speeds
Before troubleshooting, understand what speeds you should expect:
| Network | Theoretical Max | Realistic Max | Typical Achieved |
|---|---|---|---|
| 1GbE | 125 MB/s | 112-118 MB/s | 100-115 MB/s |
| 2.5GbE | 312 MB/s | 280-295 MB/s | 250-280 MB/s |
| 10GbE | 1,250 MB/s | 1,100-1,150 MB/s | 900-1,100 MB/s |
| USB 3.0 | 625 MB/s | 400-450 MB/s | 300-400 MB/s |
If you're getting significantly less than "Typical Achieved," you have a problem to solve.
Step 1: Isolate the Problem
Test Network Speed (iperf)
First, determine if the issue is network or NAS storage:
- Install SynoCli Network Tools from Package Center (Community packages)
- SSH into your Synology
- Run iperf server:
iperf3 -s - On your PC, run iperf client:
iperf3 -c [NAS-IP] - Check reported bandwidth
Interpreting results:
- iperf shows full speed, file transfers slow: Problem is SMB/NFS settings or drive performance
- iperf shows slow speed: Network infrastructure problem (cables, switch, NIC)
Step 2: Fix Network Issues
Check Ethernet Cables
Bad or low-quality cables are the #1 cause of slow transfers:
- Cat5e minimum: Required for gigabit speeds
- Cat6/6a: Required for 2.5GbE/10GbE
- Cable length: Keep under 100m (328ft)
- Damaged cables: Crimped, bent, or chewed cables cause intermittent issues
Check Switch/Router
- Verify gigabit ports: Some switches mix 100Mbps and 1Gbps ports
- Check link speed: In DSM Control Panel → Network, verify "1000 Mbps, Full duplex"
- Restart networking equipment: Power cycle switch and router
- Test direct connection: Connect PC directly to NAS, bypassing switch
Step 3: Optimize DSM Settings
SMB Settings (Most Common Fix)
Misconfigured SMB settings cause the majority of transfer speed issues:
- Go to Control Panel → File Services → SMB
- Click Advanced Settings
- Configure these settings:
| Setting | Recommended Value | Why |
|---|---|---|
| Maximum SMB protocol | SMB3 | Best performance and security |
| Minimum SMB protocol | SMB2 | Block slow SMB1 |
| Transport encryption | Auto or Disabled | Encryption adds overhead |
| Opportunistic locking | Enabled | Improves caching |
Check Background Tasks
Resource-intensive tasks slow transfers:
- RAID rebuild: Check Storage Manager for rebuilding status
- Scrubbing: Data scrub runs monthly by default
- Indexing: Synology Photos/Drive indexing new files
- Backup tasks: Active Backup or Hyper Backup running
Check Resource Monitor to see what's consuming CPU/RAM/disk.
Step 4: Fix Drive Issues
SMR Drive Problems
SMR (Shingled Magnetic Recording) drives cause severe slowdowns during writes:
- Symptoms: Fast initial writes, then dramatic slowdown (10-30 MB/s)
- Cause: SMR cache exhaustion triggers rewrite operations
- Solution: Replace with CMR drives
Failing Drive Detection
- Open Storage Manager → HDD/SSD
- Check health status for each drive
- Click drive → Health Info
- Look for Reallocated Sectors, Pending Sectors (should be 0)
Step 5: Hardware Upgrades
Add RAM
Add SSD Cache
Upgrade Network
Frequently Asked Questions
Why is my Synology only getting 20-30 MB/s?
This typically indicates: (1) 100Mbps link instead of gigabit - check cables and switch ports, (2) SMR drives hitting cache limit, (3) severely fragmented volume, or (4) failing drive. Run iperf test to isolate network vs storage issue.
Why do small files transfer slower than large files?
Small file transfers are limited by IOPS rather than bandwidth. Each file requires metadata operations. SSD cache significantly improves small file performance. This is normal - HDDs struggle with many small files.
Will upgrading to 10GbE make my NAS faster?
Only if gigabit is your current bottleneck. If iperf shows ~940 Mbps but file transfers are slower, the issue is elsewhere. 10GbE helps when your NAS can deliver faster than gigabit - typically requires SSD cache or all-SSD configuration.
Why is Wi-Fi transfer to my NAS slow?
Wi-Fi is almost always slower than wired connections due to interference, distance, and shared bandwidth. Even Wi-Fi 6 rarely achieves gigabit speeds consistently. Use wired Ethernet for best NAS performance.
My NAS was fast but recently slowed down. Why?
Check for: (1) Background tasks running (backup, scrub, indexing), (2) New packages consuming resources, (3) Drive beginning to fail - check S.M.A.R.T. data, (4) Volume nearly full (keep 10-20% free), (5) Malware if exposed to internet.
Bottom Line
Slow Synology transfer speeds are almost always solvable. Start with the iperf test to isolate network vs storage issues, then work through SMB settings, drive health, and background tasks. Most issues are simple fixes - bad cables, wrong SMB settings, or SMR drives.
For persistent issues, consider hardware upgrades: RAM improves multitasking, SSD cache accelerates small file operations, and network upgrades break the gigabit barrier. With proper optimization, your Synology should deliver consistent, fast transfer speeds.
Related Guides
- Synology SSD Cache Setup
- Synology Troubleshooting Guide
- Synology Compatible Hard Drives
- Fix Synology Drive Initialization Failed
- Best Heatsinks for Synology NAS
Last Updated: February 2026