- Wine 2.5 Staging (only version so far that doesn't make the b.net spit errors and D3 actually works)
- No Additional Wine libraries

- PC specs: (kind of a junkyard build)
AMD Phenom II 1090t
4GB RAM
Nvidia GeForce GTX 1050ti (384.59 drivers)

On Windows 10 I could max this game at around 100-120 fps on the campaign, on Linux I get what feels to be ~25 on identical settings. I'm a fresh Linux user so excuse me for any stupidity on my side.

Welcome to LQ.
If you're comparing with m$ windows, don't expect equal performance - let's get that in first.
It's probably 1) Emulation overhead & 2) Inferior video performance, and maybe 3) Incomplete implementations of windows features.
About 1 & 3 you can do very little. Let's look at 2. You can verify if this is a problem by running your game or youtube in a small window and see if it speeds up. High cpu usage is also an indicator which you can check using 'top'

Off the top of my head I can't think of a way to compare fps in wine and linux. I would start by maximising performance in linux, as wine passes that to the linux system. How is your linux performance?