With a CFExpress memory card, does the buffer ever fill, or does the camera write so fast that there is no buffer limit?
Buffer depths are also quite impressive, for both CFexpress and SD cards, though none of the shooting speeds or image quality settings allow for an "unlimited" buffer capacity rating. Given the different performance specs of the two media card types, the buffer depths can vary depending on image quality settings. Further, the maximum buffer capacity changes depending on if you're shooting at the max. 12fps or 20fps. At 12fps, the buffer capacity for Large Fine JPEGs, Large Fine HEIF images as well as Compressed C-RAW files are the same for either CFexpress and UHS-II SD cards, with up to 350 JPEG images, 280 HEIF images or 260 C-RAW files. With full, uncompressed RAW, the R5's buffer drops to 180 frames for CFexpress or 87 for UHS-II SD.
When opting for the faster 20fps electronic shutter burst mode, Canon only provides a spec for buffer capacity using the CFexpress card, which is stated as 170 JPEG frames, 83 RAW or 130 Compressed C-RAW files. However, we are told that you can shoot at 20fps while saving images to the SD card slot.