Mirrorless cameras such as the
EOS R, RP, and R5 still have mechanical shutters.
The shutter curtain transit times for FF cameras in the top classes are around 2.5 milliseconds. It's the difference between the first curtain starting to open and the second curtain starting to close that are only 1/8000 second or 0.125 milliseconds apart. But the distance between the first and second curtain is only a narrow slit about 1.2mm wide as they both transit the sensor.
(The video below is of an APS-C 7D, so the sensor height is only 14.9mm, which makes the slit for 1/8000 with a 2.5ms transit time calculate out to 0.75mm, but the way the two shutter curtains chase each other across the frame at exposure times shorter than X-sync is the same.)