Well well well, I decided instead of just wiping my entire drive & install Fedora, I’ll try a dual boot first.
So I created a separate 20 GB partition, and installed both Fedora and Ultramarine one-by-one (both Btrfs), and well, there was ZERO performance difference.
I guess I wasted the other people’s time in this thread along with mine for nothing. But, thank you for guiding this noob.
I’ve decided to install Stock Fedora 
The ease of use that Ultramarine brings is very minor & I can do pretty much all of it through a single post-install script…
What I learnt from this: Assumptions that ‘feel’ almost ‘certain’ to be true might not be always true LOL; On the live USB, fedora felt even slower than Windows 11.