Performance issues on modern hardware (latest OpenMW Nightly)

[Netchythenetch]

Hello everyone,
As the title says, I'm experiencing performance issues with OpenMW. I installed Logitech Gaming Software and I found out that OpenMW is only using two cores, which is fine for a Vanilla setup but becomes a problem when I install texture packs or mods that overhaul the world like Rebirth (I experience a significant FPS drop, and the classic "physics-through-the-roof" problem).
Is this normal or have I done something wrong?
I apologize if I posted in the wrong section.
I'm using the latest Nightly (8779d261ba). These are my PC Specs:

Spoiler: Show

Processor: AMD Ryzen 5 2600x Six-Core Processor 3.60 GHz
RAM: 16 Gb
Video Card: GeForce GTX 1660 Ti

Thank you for your time.

[DecumusScotti]

How many FPS do you get? And what are your distant land loading settings? At least on my machine (which has a weaker GPU, but comparable CPU), those settings are key. With not too many cells to load and quite aggressive small object culling, it's still quite playable, rarely dropping below 40 FPS.

[Netchythenetch]

what are your distant land loading settings? At least on my machine (which has a weaker GPU, but comparable CPU), those settings are key. With not too many cells to load and quite aggressive small object culling, it's still quite playable, rarely dropping below 40 FPS.

See, that's the thing. I don't use distant land. I'm playing at vanilla morrowind draw distance.

How many FPS do you get?

With morrowind vanilla+, my fps were 40sh outside of cities but 20/30 inside cities. Right now I'm using Morrowind Styled and my fps are consistently capped at 75 (which is perfect). I think the game runs too poorly for my hardware and I don'tkknow how to fix it. Do I have to tweak any particular setting on the ini files?
Thank you for your reply.