As for streaming, some tips (I'm a bad streamer I know, but this is the very reason I can give some advice, because I tried to improve my fingers' speed in many ways)
- ppl already wrote about hand position blah blah, also try rotating your keyboard a bit anti-clockwise. Might help
- finger workout can really help. Pianist or guitarists know it and thus they use many devices for excercise. Most of them are quite expensive, but simple kettlebell would be enough. Handgrips may be used too.
- always stretch your fingers before playing. > use your thumb and push your index finger downwards while using your other hand to bend the middle finger backwards. Then do it the other way round.
- wash your hands beforehand - sweat on keycaps may really deconcentrate
- keep your hands warm - with some workout and stretching fingers should be okay for a while. Otherwise, use hot water or if you have a microwave nearby you can sew a small bag and put some rice inside > poof! homemade hand-warmer. There're many other ways (e.g. Niko had a radiator nearby)
- If you want to train your fingers in-game, of course you can play monster maps, but there's sth else. Instead of playing maps you can barely read, you can stick to easier maps. The thing you would change is the way you push the buttons > move your fingers only! Do not move the rest of the hand. People tend to raise their palm or sometimes the whole hand because it makes singletapping far more easier and less tiring. That's why you should make your fingers tired and pressing the buttons with your fingers only is a good idea to do so. You don't have to play chipscapes or other shit, just stick to basic, 180-200bpm normal insanes containing many single notes. You'll feel and see your muscles working ^^
I'm sure there was sth more, just forgot.
also
I think it's more important to be precise on your streams before you even think about speed
so true