With a clean install of 20.04 on several machines I notice a performance improvement over 19.10. I know that the gnome shell has a lot of performance improvements, did some of these help UB?
Yes. There has been several improvements to the window manager that we share - mutter.
So budgie desktop inherits these automatically.
Likewise the kernel has made a step change in performance improvements notably to resolve some of the issues caused by previous fixes to high profile kernel vulnerabilities.