It is a very slow and painful process
My experience is that sometimes, could be in an interval of a week, but especially on higher speeds it will be more like after a year (I've been stuck at 165 BPM for a year before I advanced to 180), you have some sort of "outburst" in speed, and everything goed well and you feel really good about yourself. Then the next day you try to do it again and you can't do it anymore; sometimes it even seems you got worse. So if you're gonna go for speed, prepare for frustration, agony, pain, and a looooooong wait (maybe "wait" isn't the best word, because of course you gotta practise a lot)
And I take it you're already familliar with the principle of metronome, 16th notes, no faster than you can or you'll get sloppy?