So I've been a faithful clementine user since 2013 because it's FOSS with out of the box flac support, works on both windows and linux, and has a android remote.
As my library has bloated with more and more EDM FLAC,. I've noticed the app becoming less responsive, especially when searching. I have a Ryzen 7 and an NVME SSD, and my RAM usage never went above 60% so I don't think it's a hardware limitation.
I vowed never to muck about with processor priority after breaking things once before.
Can someone recommend an app that has snappy search even with 300gb+ libraries loaded in its search function?
Learn to setup music player daemon, MPD. Its not simple compared to most music players but its basically the final god-form of music players. You'll need to read the manual to set it up but the final config will only a little over a dozen lines. I did have to fuck with some /etc file for libasound but thats pulseaudio being a prick as usual.
If you get it setup there's no going back. Pick and choose your player to control it. Most foss players can be set to use mpd as a back end, even celmentine has it built in.
Also if you have 300gb of music you should look into beets to sort and index it. It'll be a rough start to initialize the database but it'll be faster than doing itanually. Having an auto renamer when dealing with that many files is pretty clutch. I think some players can use beets as a backend but i just use it for renaming files.