I don't want to highjack berta's thread, but my case is super identical.
I have nvme and ssd cache drives (two separate single disk caches) and a 10 gig network (both servers and desktops are with 10 gig NICs).
After upgrading to 6.10.3 my speeds are 100MB/s fixed, no more, no less.
It does not matter if I copy from or to the cache drives (tried both of them in both directions).
Reporting on Unraid's MAIN tab is also weird.
Tranfering 1.5TB of data from my Win 10 Desktop PC to the server (cache drive is empty and 2TB in size).
On the Unraid > Main > Pool devices > cache_ssd > write column ... the speeds reported are 0 MB/s for some time, then 300-450 MB/s ... then 0 MB/s again ...
Same behaviour can be observed with smaller 3-4GB files.
Before the Unraid version change the speeds were close to the maximum supported by the drives/network.
The only change in the setup was the new version.
Hopefully it would be fixed in some future upgrades, because I hate the idea of downgrading the OS.