I was thinking about grabbing Crimson Dragon as it's just $5 now and I do kinda like rail shooters.

For those who have/had an Xbox One, is it normal for the UI/OS to be very slow and laggy, or is there something I can do to fix it/speed it up?

Break it open and install a SATA SSD... You can install an OS via a USB flash drive.
It's what I did for my Xbox One X and Xbox One VCR with Kinect; it's a transformative experience with almost everything... Reduced load times, less noise and some games manage textures and asset streaming better as well... But the UI is much much much improved.
Microsoft just didn't give the OS enough memory to play with for a seamless experience... So it streams data from a swap file from the hard drive... Which is a common issue with consoles in general. (Think: WiiU, Switch, PS3)

Ah gotcha; that's a bit out of my wheelhouse as I am hopeless with anything technical or hands-on, but thanks.

I wasn't expecting it to be as fast as a Switch 2 or PS5, but it did surprise me that it's slower than my Xbox 360 despite being 8 years more modern.

Yeah, She is a clunker.

Part of that was in Microsoft's vain attempt at cutting back on UI/OS features to make the console more performant where it matters... In video games.

As for upgrading the internal drive, it's a super easy process, just remove the bottom screws and swap the Sata HDD for a Sata SSD.

I imagine those old Jaguar CPUs don't help matters either.

Still, I'm not super fussed over performance, I survived 4 years of the Wii U which was slower than a stoned tortoise, and graphically Switch 1's been my most played system from 2017 to this June, and I play 360/Wii/OG Xbox/etc fairly often so I can handle the games looking a bit rough.