Canon EOS R3 Advanced User's Manual page 339

3.
Focus and record the movie.
Same as steps 2 and 3 for Autoexposure Recording.
Caution
Avoid adjusting shutter speed while recording movies, which will record changes in
exposure.
When recording a movie of a moving subject, a shutter speed of approx. 1/25 sec.
to 1/125 sec. is recommended. The faster the shutter speed, the less smooth the
subject's movement will look.
If you change the shutter speed while recording under fluorescent or LED lighting,
image flicker may be recorded.
339