I was having a lot of problems with performance in my environment until very recently. I couldn't quite put a finger on when I started noticing things getting really bad but it was definitely getting worse over the past year. Then, a couple of days ago, I stumbled upon some things that helped me to resolve my slowness problem on Mac. Essentially, for the past several years the performance of SMB has gotten worse and with most of us trying to tinker with settings to improve performance, which many of us did, it would seem that around last year, limetech actually decided to work on improved settings for SMB... which after looking at this post here, caused me to figure out that all of my settings and changes combined with their settings and changes was actually the culprit. And, interestingly enough, that was around the time that I started to notice performance getting really bad for me. The fix for me was to remove all of my SMB extra settings completely, copy the fruit conf file that is in the post, add my appropriate changes that I wanted to be default, removed the ones that they explicitly state shouldn't be working, and restarted samba. When I tell you performance was night and day, it would be an understatement. To confirm that was the issue, I undid everything and put it back to what it was before and sure enough, performance tanked. To put into perspective, what would take only a few seconds to copy on my 10gb network was taking hours. Now, things are back to taking seconds.
My advice would be that if you have made your own settings changes to unRAID over the years in order to improve performance, especially in the smb extras files, if you are running 6.11 or newer, you may want to re-evaluate your settings against the default, as you may be inadvertently negating options designed to improve performance from some of the changes put in. That is what was happening to myself.