Hi,
This is tradeoff between video quality and performance. That is the reason MEDIUM and FAST do not have identical performance. A possible solution is to set to FAST and tune virtual buffer size. Please refer to this topic:
Random blockiness in the picture RTSP server-client -Jetson TX2 - #5 by DaneLLL
So if you can run in FAST preset mode and adjust virtual buffer size to get good video quality.