I would disagree with what Halogen- would say about speed and jacking ability being somewhat intertwined and would say these things:
- Speed (particularly stream and light jumpstream speed) is extremely heavily based on reading difficulty. Exposure to fast streams/light jumpstreams would help substantially with your reading, which in turn would help improve your speed in general. Try to target shorter streams/light jumpstreams that are a step outside your comfort zone to orientate yourself (I can't give any because I don't have a speed range) and then play longer ones to master said speeds. You do not need that much physical speed (and tension in general) as you might think for fast streams/light jumpstreams in comparison to handstreaming or jacking. Play those and you'd help with reading, as long as you know a good technique to discern patterns accordingly. There are several ways of doing so, but it'd take way too long for me to break down every single way of reading streams.
- Handstreams (streams with lots of triples and doubles) is partially reading difficulty, but it's generally a very physically straining pattern more than anything else. Main reason for this is due to the huge abundance of one hand trills and two hand trills, which creates considerable tension that most patterns don't create. Play those as well and you'd be able to build one hand trilling ability pretty quick. Two hand trilling in handstream is generally not quite as abundant, you can also improve your jacking ability as well but chances are if you can't do handstream at a certain speed despite your one hand trilling being solid, you'd have to work on your jacking instead.
- Jacks are unarguably the easiest pattern to read but it's also one of the more taxing patterning in terms of physical ability. It really comes down to how well you can optimise your technique for jacking. Don't tense up unless you really need to (at a certain point you have to tense up, but your goal is to ensure that you rarely tense up, as tensing up drains your stamina at a considerably faster rate and you end up with locked fingers). Being able to hit fast jacks takes some practice on its own, but it's just playing them to the point that you figure out a good technique to hit them or the speed of the jacks just enter your comfort zone.
Reading, jacking and one hand trilling is all there is to 4K. They are skills that can be trained individually and you'll improve substantially in maps that heavily test said skill if you do, but it's best to work on all of them as all of these skills will be tested in a single map, just to different extents. Jacking and reading goes hand-in-hand pretty often for most maps here (at least from what I've experienced on DT), but for SM converts it's mainly reading and one hand trilling. SMOC converts are an exception, but those charts practice a certain type of jacking, mainly fast short jacks. From what I've seen, it just comes down to how much you practice maps that are heavily testing jacking or one hand trilling or whatever. It's just that most of the players Halogen- mentioned play a good number of maps that test all three skills, with different emphasis on each. Bottom line, just practice those three types of skills by exposing yourself to as many maps as possible.
Also disregard what Gabe said because it is pseudo-elitist toxic dribble. Expose yourself to many styles of mapping, expose yourself to a myriad of maps and you'd be able to become a well-rounded player.
, that have really fast streams/stairs/ln's. Some have told me that I should change my skin, which is based on f1rst's skin with those really slim notes, but whenever I try to use normal size note skins, I lose a lot of accuracy since I got used too much to my current skin, but with hell-fast patterns everything becomes very blurry 
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