I am shocked.

I am running a Linux Mint 21.3 (Kernel 5.15.0-122) system on a core i7-14700/GeForce RTX 4700 (nvidia-driver-550) machine and when I start a new game with 10 colonists on the default 250x250 map I get a maximum of 11 FPS right away.

Rimworld 1.5.4104 rev415, no DLC, no mods.

What's up with that?

Do you see any messages in your terminal if you start the game from there?
That might give you a clue what's amiss, provided that the packager/ developer left those messages in.
If you are running the game from a laptop, the game might be running on the UHD 770 Intel GPU Core instead.

Sorry for the late reply.

Thanks Jorlin, you set me on the right path.

The output of the terminal led me to update the MESA driver, which led me to question the GPU drivers, when that brought no improvements.
Lo and behold:

The proprietary nvidia drivers were the root cause of my ills.

I switched to the open source driver package and now the game really has some legs.

Trying a run with everything that says "Vanilla expanded" on the tin now. :)