Anyone having SMB performance issues with Unraid, especially with user shares should update to v6.11.0, as mentioned in the release notes it includes Samba 4.17, and from the Samba release notes:
I did some tests (see below for more details of how I do the testing) and SMB performance improved a lot when writing small files to a user share, which is where I've been noticing the biggest performance loss with each new Unraid release, I believe that the biggest part of that loss was from the Samba security related changes, without the new Samba v6.11 was on track to be again much slower than the previous release, -rc5 still uses Samba 4.16:
Another thing that looks responsible for a considerable performance loss are the Spectre/meltdown mitigations, especially on later kernels, though this can vary with the hardware used, mostly CPU/board/BIOS, it's my understanding that for home users, and unless you are renting VM space, it's extremely low risk running the server with mitigation disabled, and you can do that by adding mitigations=off to syslinux.cfg, or use the existing plugin, note the performance difference with both small and large files especially with both v6.11 releases, likely because the kernel now also has mitigations for Retbleed, also note the difference in the find test, where it's just the time it takes to list 2500 files over SMB.
With the new Samba release and mitigations disabled performance is a good as I've seen for many years, though v6.7 and v6.8 are still faster in some cases, likely from other Samba/kernel changes over the years, looks like any performance loss is inflated by FUSE, so what might have been a small loss turns into a much bigger one when using user shares.
Tests are done using two Unraid servers running the same release for each test, small files test uses around 10K small files of various sizes that total around 10GB, large files test uses 9 large media files totaling about 40GB, each test is run 3 times by doing a timed cp command where I take the time in seconds it takes (the "runs" column) and from that the average speed is calculated, 500GB Samsung 980 NVMe devices assigned to a pool are used as the source and destination devices, find test is just how long it takes (average of 3 runs also) to list the contents from a folder with 2500 files over SMB using a disk share and a user share.