So, I've been trying to use various Raspberry Pis as Steam Links for quite a while now, but they all seem to have the exact same problem. Whether I'm trying to use them wired or wirelessly, after playing any game for a minute or two -- which is actually perfect for that minute or two -- there's an enormous lag spike, the entire screen gets pixelated, and it seems to have to take a few seconds to fix itself before everything goes back to normal. Every single game that I've tried does this, and I've tried multiple games on a Raspberry Pi 3, 3 B+, and 4 B. Wired or wireless, resolution high or low, it doesn't matter, it happens with everything. I've tried changing every setting you can imagine, both host- and Pi-side, but nothing seems to fix the problem.

Here's my current build. For what it's worth, the actual Steam Link hardware (the discontinued unit) is flawless, and the Amazon Fire TV stick seems to work pretty well, too.

https://pcpartpicker.com/list/TR7zdD

EDIT: Okay, so, it's definitely the Raspberry Pi OS that's causing the problems. I formatted my Pi's micro SD card and put Retropie on it instead, and after installing Steam Link on there, it's flawless.