Very easy to reproduce: 1. Buy cheap QLC drive. 2. Fill with Steam games. 3. Delete some steam games and download new games. 4. Watch write speeds tank to zero for long periods when downloading.

It's due to garbage collecting on very slow QLC NAND. You won't see it until the drive starts to get 60%+ full. Until then, the drive pretends it is an SLC with very fast writes, but then it starts to show its true colors. Yuck.