[Gameplay] Framerate drops as game approaches ~30 minute mark

For the last couple of months I've been noticing that in all of my games, the framerate starts out around 60 fps, lowers to 40-50 when minions spawn, but then start approaching 20 fps and getting framerate drops as the game gets to roughly 30 minutes. Restarting the game fixes this. Using lowest graphics settings doesn't prevent this from happening.

Steps to reproduce:

1. Play a game for ~30 minutes
2. After the framerate is unbearable restart game

Frequency: every game.

I just played a Bard One-For-All and this effect happened a lot faster. Maybe it's related to accumulation of previously spawned chimes or something, I don't know.