It was with half battery: Canon's ES fps doesn't slow down with battery drain. The SD card is more than fast enough for a 21 shot burst - the cache on the R5 is about 2Gb or 40 shots of full RAW. But, that's beside the point - the camera has a CFExpress slot. Complaining that everything has to be right to get maximum performance is like complaining your car doesn't get the stated mpg when driving up hill.
Canon indicates that their OEM batteries will give you top frames per second until they're about a third used up. BUT, tests show that they actually provide that performance for two thirds of their capacity. Third party LP-e6NH batteries do indeed have the full-performance for only one third of their capacity.
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