If you want a way to practice this outside of just... Well... Play more. Try and find an online metronome with a customizable speed (There are some that have a few set speeds, I know, but I wouldn't suggest using them), pick a BPM, and alternate your keys to the beat as if you were playing straight eighth notes (sixteenth notes are quicker, but for the not-so-rhythmically-inclined, eighth notes are a lot easier to wrap your head around), once you have that rhythm down, switch to another, faster or slower, whichever, BPM and do the same. Keep doing this until you can switch between high BPMs without a problem. Important note: NEVER MASH! If you want 50s and 100s, sure, you might be able to get by with mashing your keys as fast as possible, but it definitely pays off a lot more to actually be able to figure out the rhythm instead of just switching to "Alternate as fast as I can" whenever any stream comes up.