My main library is 3,808 files, 52.5 GB. There are a lot of songs and soundtracks, and roughly 24.5GB is flac (833 files).
I switched from iTunes to MusicBee at the start of 2017, but back then I had no flacs, and maybe half or a third the number of files. MusicBee was by far superior in speed (starting up, running, processing/tagging). It's always had the tiniest bit of unpredictability in whether it starts on command (might take a couple clicks of the icon before it chooses to display the start-up).
I have it sitting in the background 99% of the time in Compact Player view, having assigned CTRL+SHIFT+M to switching between compact and main view, because I do this frequently.
However, since I have recently been steadily adding to my library from what was once 10GB to >50Gb, I have noticed that when I switch from compact into main view, it has taken longer to properly display (windows stays white for a couple seconds while it prepares itself.
I am also not sure if this is the doing of my larger library or the major update that I installed (from mid-2017 to August 2018 I did not update).
Also, recently it has been crashing more frequently, usually if I'm editing a large number of tags or converting a large number of files.