Question RX580 - Poor performance on Windows 10

Dec 29, 2020
-Gigabyte GA-Z270-HD3P
-Intel Core i7 6700k 4Ghz
-Ballistix Sport 16gb DDR4 2400MHz
-EVGA 600 B1 80+ Bronze 600w PSU
-MSI Armor RX580 8Gb OC

Hello everyone, first time posting here.

I recently acquired a new GPU (mentioned in specs) and it is seriously underperforming. It's doing much worse than my previous GPU, an EVGA 1050ti.
Games feel choppy; some are worse than others. For instance I wanted to try out Black Mesa with this card and it gets an average of ~30fps compared to almost constant 60 with the previous card.
I'm pulling my hair out trying to figure out what's wrong with it. Spent hours tweaking in the Radeon software; no luck. Fresh Windows install, fresh driver install, playing around with different Windows power settings - nothing.
The temperatures seem to run a tad bit hot (bad case design) at ~42C in non-demanding games (Minecraft?) I can't imagine this would be the reason for my issues. I've read that these cards will run well up to 90C.
I've tried mostly everything I could find on the web so far.

Now, here are some important things to note:
1 - I'm using Windows 10
2 - This computer is a hackintosh build. I tried some other games using the Mac OS and to my astonishment - they all run flawlessly on that platform. Which tells me that the GPU is perfectly fine; it's most definitely some kind of software issue on the Windows 10 side of things.
3 - The card has performed poorly using the PerformanceTest program. The DX12 test averages around 90FPS (or so it says - but it looks to be far less than that.) Not long ago I attempted a "clean boot," disabling all startup services and whatnot. It might have been a fluke, but I decided to run the DX12 test again immediately after boot, and for a few moments I got a brief performance burst - the same test ran at around 190FPS - looked much smoother. Then it immediately stopped and dropped back to where it was before. I haven't been able to replicate this since.

My current theory - some unknown Windows 10 process running in the background is just completely tanking the performance (and during the miraculous DX12 test I managed to get things running before said program started up and had the chance to interfere). But nothing in the Task Manager indicates this. I have no clue where to go from here. Was considering doing a test install of Windows 8.1 as a last resort, to see if that operating system handled the card any better.

I would really appreciate any and all help or suggestions; I don't think this is a bad GPU and I want to get it working the way it's supposed to. Sorry for the mouthful.

Dec 29, 2020
An important update:
-I've found that the card performs properly only when running games in windowed borderless mode. For some reason fullscreen mode causes significant performance drops.
This is still a problem because many of these games don't include a windowed borderless mode. What would be the cause for performance drops in fullscreen?