Weird. I have sometimes around 100 FPS and it is stuttering like crazy (with or without Vulkan and on/off GPU Skinning, nothing helps). I basically can't play till a fix comes.
I didn't have any mods. It was a clean install. But today I did an update and Arch downloaded:
"5 extra/0ad-data a26-2 -> a27-1"
now it works. So before I had a27-1 with the data of a26-2? Weird. Anyways, I am glad it is fine now.