Keeping in time with streams, hands down. Like, I'm playing a jump map and doing completely fine, then one stream appears (not even a hard stream, 170-190 of like 8 or so beats) and I end up getting a miss. Whenever I'm hunting down scores to beat by looking at other peoples userpages, I always find stream maps completed by people of a similar rank to myself that I could never beat, but the maps without streams I can usually challenge their scores on.
I'm trying to improve it right now, I'm learning hit meter and basically am specifically playing stream maps for the time being. I've felt an improvement in the past week from playing the deathstream compilation over and over, finally passed mythologias end. So I am making some sort of improvement.