I agree with you to an extent, though some might quibble over the term "perform." The benefits of using an EF lens on the R include the near WYSIWYG exposure, the better AF for static/barely moving subjects (which includes the enhanced AI Servo making f/1.2 easy now), and, for those so inclined, the excellent alignment triangles for manual focusing.
There are two negatives, however, that might fall under the term "perform." Most obviously, the ergonomics. The size and weight of the R, combined with the extra length given to lenses by the adapater, do make a significant difference in how certain lenses handle. Personally, I switched to the RF 70-200mm f/2.8L IS because, on the R with the adapter, I could not hold it properly in portrait mode. Of course this affects different people in different ways, but ergonomics are assoicated with performance.
The more subtle performance issue relates to High-Speed display, which Canon says "is more responsive, making it easier to follow fast-moving subjects." High-Speed display is only available for RF lenses on the EOS R. (Page 149, User Guide) Will this be the same on the new bodies? If not, will better processing make up for it to a significant degree?