If the your memory card is a UHS-I compatible memory card, I recommend trying a Speed Class 6 or Speed Class 10 memory card that does not use this standard. The camera is will accept cards using the UHS-I standard, however it is not compatible with it. If this is the case, it may "see" the card as a Speed Class 2 memory card, which is too slow for the recording of high definition movies.
I have seen some SD cards underperform due to issues with formatting. After backing up all the files on your card, you can format it with the SD card format utility (free) on your mac or PC. Then format it again in camera (low-level format).
This method has cleared up a lot of issues I've had with cards not being able to write at their rated speed.
12/05/2023: New firmware updates are available.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.