Relayer

This may be the first time that aftermarket boards may have trouble keeping up with the cooling of the reference boards. I can't wait to see how it develops.

Interesting, I've seen a few posts on other sites recently regarding thermal paste drying out on AMD 480 and newer cards. I would check that out, I have 2 480s here, one on shelf and another in backup computer I plan to redo soon.

It could be just Windows Defender running at startup, indexing, etc.

Nice computer, it should run anything you want if you can keep everything cool. If it were me I would change that CPU cooler out for something better. The video card itself should be just fine, if you can get cool air to the fan intakes, which are located on the bottom of the card just above the power supply shroud. Not much of a gap there, 1.5 inches maybe? Looking at the front fan it's not getting any air into that gap from the looks of it so the video card probably is just recirculating air, getting hotter and hotter, not cool... (sorry) Is there provision for mounting an extra front intake fan? If not, change out that front fan for a high flow version. Put 2 in if at all possible, a fan in the lower position looks to have a decent chance of getting air into that gap. I would also change out that rear exhaust fan for a high flow version. This looks like one of those cases (couldn't help it again) where the power supply shroud is restricting air supply to the video card intakes pretty severely, remove it if needed. You'll have to look at those ugly cables (oh no!) but it should open up air movement quite a bit. Good luck!

Is it throttling due to temp? Check cooling fins, case airflow, etc.