SnapRAID has no effect on filesystem or system performance unless an actual SnapRAID job is running on otherwise inadequate hardware.
You can rule out unionfs as a source of the problem if you temporarily create a new library in Plex that doesn't use the union as a media source. Just add individual drives instead. If the buffering remains, look somewhere else.
I have 40TB in a mergerfs union spanning eight drives and Plex has no problems with it. The CPU is a four core Intel Atom 2550, rather meager by today's standards.
I just now saw that you are on Emby, not Plex. But that doesn't alter my suggestion about using drives instead of the union as a test.