My EF 400 2.8 L IS II doesn't get near the FPS in mechanical shutter as my RF 28-70 f2L and the 400 isn't all that old. If you think all of your old EF lenses are going to work perfectly on the R3 you're delusional because it's physically impossible due to the lack of electrical contacts compared to the RF's.There are only a very limited set of lenses that have reduced functionality with the adapter, and they're mainly really old lenses. The early 90's EF 35-80mm PZ lens doesn't power zoom when adapted to an R-series body, and the AF Stop buttons on the MkI IS 'great whites' (200/2, 300/.8, 400/2.8, 500/4, 600/4, 800/5.6 and 400/4 DO) don't work via the adapter (source).
However, there are some notable body limitations with EF lenses. There are several 'popular' lenses, albeit older ones, that don't support 12 fps on the R5, e.g. the 100-400 MkI, the 85/1.2L II, etc. Those lenses could do 16 fps on the 1D X III, but are limited on the R5. As another example, the R5 and R6 (and presumably the R3 will) offer focus bracketing – that works with all the RF lenses but only with four EF lenses and three EF-S lenses (most of the macro lenses are on the list, but not the 100mm non-L and not other lenses with reasonable magnifications, like the 100-400 II