How to Troubleshoot Slow File Transfers on a Local Network
Your local network should transfer files at 90–115 MB/s on gigabit ethernet. If it is significantly slower, one of a handful of specific things is wrong. Here is how to find and fix them.
Healthy wired gigabit should move files at about 90–115 MB/s. If yours is far slower, work through it in order: measure the real speed, check the negotiated link speed (a port stuck at 100 Mbps caps you at ~11 MB/s), swap the cable, rule out an old or oversubscribed switch, and check whether a slow hard drive on either end is the actual limit. It is almost always one specific link in that chain, not the whole network.
What You Should Expect
Before diagnosing slow transfers, establish a baseline expectation:
| Connection type | Expected transfer speed |
|---|---|
| Gigabit Ethernet (1GbE) | 90–115 MB/s |
| 2.5 Gigabit Ethernet | 270–300 MB/s |
| WiFi 5 (good conditions) | 30–80 MB/s |
| WiFi 6 (good conditions) | 60–150 MB/s |
| Fast Ethernet (100 Mbps) | 9–11 MB/s |
If your network transfer speed is significantly below these figures, something specific is wrong. The most common causes are well-understood and usually fixable.
Step 1 — Measure Your Actual Transfer Speed
Open Task Manager during a transfer: Ctrl+Shift+Esc → Performance tab → Ethernet or WiFi
The send/receive rate shown here tells you the actual network throughput in real time. Alternatively, divide the file size (in megabytes) by the transfer time (in seconds) to get MB/s.
Is the measured speed close to what you expected?
- If yes: the network layer is fine. The bottleneck is likely storage (see Step 5).
- If no: continue to Step 2.
Step 2 — Check the Link Speed
The first hardware variable is the link speed negotiated between your network adapter and the switch.
On Windows:
- Right-click the Start menu → Device Manager
- Expand Network Adapters → right-click your Ethernet adapter → Properties
- Go to the Advanced tab → look for Speed & Duplex
- Confirm it shows 1.0 Gbps Full Duplex (or 2.5 Gbps if applicable)
Alternatively: Settings → Network and Internet → Status → your Ethernet connection → shows the link speed.
If it shows 100 Mbps instead of 1 Gbps: The link has negotiated down. Causes: a bad cable, a damaged port on the switch, or the adapter's auto-negotiation failing. Try a different cable and a different switch port.
On Mac: System Settings → Network → your Ethernet connection → Details → shows "1000baseT" (1 Gbps) or "100baseTX" (100 Mbps).
Step 3 — Eliminate the Cable
A damaged or substandard Ethernet cable is one of the most common causes of slow or unreliable gigabit connections. Cat5e and Cat6 cables are required for gigabit speeds. Cat3 and Cat5 (not Cat5e) cables are limited to 100 Mbps.
Intermittent physical damage — a kinked cable, a cable run near power lines, a connector that is not fully seated — can cause the link to negotiate to 100 Mbps or cause packet errors that reduce effective throughput even at gigabit link speeds.
Test: Swap the cable with a known-good cable and retest transfer speed.
Step 4 — Rule Out the Switch
A failing switch port can cause the same symptoms as a bad cable.
Test: Move both machines to a direct connection using a crossover cable (or a modern cable, since modern NICs support Auto-MDI/MDIX), bypassing the switch entirely. If speeds are normal without the switch, the switch is at fault.
Also check: are both machines on the same switch, or is traffic routed through a router? Router-mediated traffic can be slower if the router is CPU-bound by NAT or firewall processing. Two machines on the same switch should transfer at full switch backplane speed without router involvement.
Step 5 — Check Storage Speed
If the network layer is performing correctly but transfers are slow, the bottleneck is storage — either the source drive (where the file is being read from) or the destination drive (where it is being written to).
Check drive read speed: On Windows: run CrystalDiskMark (free) to measure the read speed of the source drive. On Mac: run Disk Diag or Blackmagic Disk Speed Test.
Typical drive speeds:
- 5400 RPM HDD read: 80–120 MB/s
- 7200 RPM HDD read: 120–170 MB/s
- SATA SSD read: 400–550 MB/s
- NVMe SSD read: 2,000–7,000 MB/s
If your drive reads at 100 MB/s, you cannot transfer significantly faster than 100 MB/s regardless of network speed. Simultaneous reads from multiple sources, or writing to a slow destination drive, can reduce this further.
Step 6 — Check for SMB Protocol Overhead
If you are transferring via Windows SMB (shared folders, mapped drives), the SMB configuration can affect throughput.
SMB encryption: If SMB encryption is enabled (not the default on standard Windows installations, but possible on some managed or enterprise configurations), encryption adds CPU overhead that reduces throughput on machines without hardware AES acceleration.
Check SMB encryption status: Open PowerShell as Administrator:
Get-SmbSession | Select-Object -Property *
Look for EncryptData — True means encryption is active. If encryption is enabled and transfers are slow on older hardware, consult your IT administrator.
Large MTU (Jumbo Frames): If your switches support jumbo frames (9000 MTU), enabling them on all machines on the same segment can improve sustained throughput for large files. All devices on the segment must have matching MTU settings for this to work. Partial jumbo frame deployment causes fragmentation and can make performance worse.
Step 7 — Test With a Different Tool
If SMB transfers are slow, test a direct file transfer using a tool that bypasses the SMB stack entirely:
- LocalSend (free, cross-platform): uses HTTPS over LAN
- Oxolan (Windows): uses its own transport layer
If a non-SMB transfer achieves much higher speeds, the issue is specific to the SMB configuration (permissions check, antivirus scanning, encryption overhead). If the non-SMB transfer is equally slow, the bottleneck is at the network or storage layer.
Common Culprit: Antivirus Real-Time Scanning
Antivirus software that scans every file being written to a destination can reduce write throughput by 30–60% depending on file type and scan depth. During a large transfer, every file passes through the antivirus engine before being written to disk.
Test: Temporarily pause real-time protection (with appropriate caution) and retest transfer speed. If speed significantly improves, configure your antivirus to exclude known-trusted network shares or local NAS mounts from real-time scanning.
Frequently asked questions
Why are my local network file transfers so slow?
Usually one specific cause: a port negotiated down to 100 Mbps (caps you near 11 MB/s), a damaged or old cable, a cheap or overloaded switch, a Wi-Fi hop in the path, or a slow hard drive on one end. Measure the speed, then check link speed and cable first — those two explain most cases.
How fast should file transfers be on gigabit Ethernet?
About 90–115 MB/s for a real file copy on healthy wired gigabit. If you are seeing 10–12 MB/s, a link has dropped to 100 Mbps somewhere. If you are seeing 30–50 MB/s, suspect a Wi-Fi leg, an old cable, or a slow disk. Consistent results near 100 MB/s mean the network is fine.
Is my slow transfer the network or the hard drive?
Run iperf3 between the two machines to test the network alone. If iperf3 is fast (~900 Mbps) but a real file copy is slow, a hard drive is the bottleneck, not the network — common with older spinning disks or a nearly full drive. If iperf3 is also slow, the problem is in the cabling, ports or switch.
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