Just upgraded from MB2 about four days ago.
My wife complained to me that video was randomly becoming laggy and stuttering for short periods. I noticed it myself and had a look. Looks to me like whenever the library scan kicks in the performance drops way down. I noticed that everytime a scan occurs it starts hitting the internet for metadata, but MBS appears to ignore the [dontfetchmeta] tag and everytime a scan occurs it requests metadata for the same stuff it failed to find last time it ran. Runs for about 5-10 minutes and then performance picks back up.
I disabled getting metadata from the internet and this solved the problem, but makes me sad. :-(
Currently running MBC and MBS on same machine.
MB-Classic 126.96.36.199 B11-3.1