Alpha 27 of 0 A.D. has a notable performance regression for a subset of users, compared to earlier versions. Users not affected by this regression should see improved performance, thanks to improvements like the added Vulkan support.

The regression in performance seems to be caused by a change in Spidermonkey. For details check out https://gitea.wildfiregames.com/0ad/0ad/issues/7714

That said, even without this performance regression, 0 A.D. is prone to run slow in late game when lots of units are on the map. There are several reasons for that, but maybe the most intuitive one is that 0 A.D. is still largely single-threaded and therefore doesn't make use of the multi-core capabilities of modern CPUs. As you can imagine changing that is no easy feat and takes a lot of effort. As the number of volunteers to 0 A.D. is limited, nobody has picked up that topic yet.

If you enjoy 0 A.D. and want to improve it: it's Open Source and contributions are always welcome!