guide · static export
PC

Find Out Why SMB File Copies Are Slow

Measure real throughput, read the negotiated dialect, and check the four settings that throttle a gigabit network.

by@guidox updated:10/31/26 time:30 min level:Intermediate tested on:Windows 11 24H2 works on:Windows 10, Windows 11 tools:PowerShell, A share on the LAN
TL;DR

Copy with **robocopy /MT** while watching Task Manager > Ethernet, then read the dialect with `Get-SmbConnection`. A gigabit link should carry about 110 MB/s; 40 MB/s points at a setting, not the cable.

Before you start

Requirements

  • A share on the LAN
  • Admin rights on both ends
  • Thirty minutes
Step by step

Instructions

01

Baseline with one large file

Small files measure the protocol overhead, not the link. Unbuffered I/O keeps the file cache out of the result so the number reflects disk and network together.

robocopy \\NAS\share C:\Temp\Disk testfile.iso /J /NP
02

Measure with parallel streams

Many small files need multiple streams to fill the link. One large file cannot use them, which is why the two tests disagree.

robocopy \\NAS\share C:\Temp\Disk *.* /S /MT:16 /NP
tipTask Manager > Performance > Ethernet shows the same rate the copy reports; if it does not, the disk is the limit.
03

Read the negotiated dialect

SMB 3.1.1 encrypts and signs efficiently. A 2.x dialect means old firmware or a router inspecting SMB, and both cost throughput.

Get-SmbConnection | Select-Object ServerName,Dialect,NumOpens
04

Check signing and encryption

Encryption costs processor time on both ends and on a weak CPU it halves the rate. Signing is cheaper and usually enough on a trusted LAN.

Get-SmbClientConfiguration | Select-Object RequireSecuritySignature,EnableSecuritySignature
⚠ warningTurning encryption off on an untrusted network trades speed for exposed data. Decide on the network, not on the benchmark.
05

Check the physical link

Half duplex, a marginal cable or a 100 Mbit port produce exactly the 10 to 12 MB/s plateau people blame on SMB.

Get-NetAdapter | Select-Object Name,LinkSpeed,FullDuplex
06

Exclude the folder from real-time scanning

An antivirus scanner reads every byte written and every byte read. On a file copy that is often the entire difference between 60 and 110 MB/s.

Add-MpPreference -ExclusionPath C:\Temp\Disk
tipExclude only the test folder. A whole-drive exclusion is a real risk, not a performance tip.
Troubleshooting

Common issues

The copy starts fast and drops

The write cache filled up. The steady rate after a minute is the real one.

The client is on Wi-Fi

80 MHz on 5 GHz gives 40 to 60 MB/s in practice. Cable it before changing any setting.

Only one direction is slow

The receiving disk or its write caching policy is the limit, not the network.

The dialect reads 1.5

SMB1 is enabled on the NAS. Move the device to SMB2 or SMB3; SMB1 should not run at all.

FAQ

Frequently asked

Do jumbo frames help?

Only when every device on the path supports them. One switch at 1500 MTU makes throughput worse, not better.

Does SMB multichannel need two network cards?

No. Receive side scaling on a single card also spreads the load across queues.

How to undo this

Remove the antivirus exclusion and restore signing or encryption in the same dialogs if you changed them for the test.

Changelog · 1 entry
  1. 2026-10-31
    First version
Interactive version: checklist, feedback, related guides →

Free · no account needed · tracks your progress