There was another topic on this, SMB and MacOS Finder speed, again
My experience is that the macOS Finder will walk the tree gathering metadata when it first opens (I suspect it actually does this in the background once the share is mounted, but I have yet to test that). If the number of items is large, the time to gather the metadata is noticeable.
How many items (files + directories) are there at the root level of your shares?
Do you see slow copy times for large items copied via CLI (terminal)? via Finder?
When I was running CORE I was able to speed up the initial opening of the share by doing 2 things. I used AFP instead of SMB and I configured the AFP share to put the CNID database on a very, very fast SSD (Intel Enterprise with over 50,000 I/Ops performance for small, random reads and writes).
I do not (yet) have a tuning for SCALE and SMB to minimize the wait while the Finder gathers metadata.
But, I had run performance tests of SMB vs. AFP on macOS to TrueNAS CORE and found they had comparable read and write performance for large (greater than 1MiB) files.