I've attached four shots taken at 1/100th of a second, f/2.8, ISO 640. Window light. The file names indicate which ones were taken with EFCS or Mechanical, IBIS or no IBIS.
They were all taken with an Rf 24-70mm f/2.8L IS at 35mm. I found the same results at 24mm, but didn't test at a longer Focal Length. I also found similar results, but not as pronounced using an ef 35mm f/1.4L at the same settings.
I took the same shots at 1/200th and found that, at 100%, the shutter shock was barely detectable, not a real world factor, but if I zoomed in to 200%, I could see the tiniest bit of difference. Really not any issue, imo.
I did the same tests at 1/30th of a second (ISO 160), and I've attached two of those to show that there is still visible shutter shock, though not as bad as at 1/100th. Both were taken with NO IBIS because the results were pretty much the same with it on, and because I think most photographers at 1/30th are going to choose no IBIS on a tripod. I do know that 1/30th is in the range dSLR shooters would use mirror lock-up, but I will admit I thought the blur at around 1/30th in dSLR's was caused by the mirror. I guess the mechanical shutter is involved there too.
For myself, I have to put some perspective on this. The R5 shows shutter shock at 1/100th of a second with mechanical shutter. This is aggravating because I tend to shoot quite a lot indoors from 1/60th to 1/160th, and I'm USED to mech shutter, I trust it. But I have been taking photos under LED with EFCS, and I'm not seeing any drawbacks in my images. So all I have to do is remember to switch to mechanical outdoors, bright light, wide apertures to avoid the bokeh problems. Am I missing any EFCS drawbacks here that might bite me someday?
Another perspective that helps me deal with a less than absolutely perfect camera, I know that shutter shock has been a quiet issue for decades, and that we learned to work around it when shooting in that slower shutter-speed danger zone. I can still work around it easily here, but I just have to remember which Shutter Mode to use in various situations. I think for the most part, as a portrait and events photographer, I can use EFCS most of the time. Bright sunshine and higher shutter speeds, go to mechanical.
Can this be fixed with a firmware update? If not, would Canon make some change in production to improve the vibration issue? If so, where would that leave us early adopters?
(I'm really getting behind on yardwork this week, and it's only Tuesday! Cheers!)