If your arm starts to hurt from playing, you are probably doing something very wrong. If you compare smash to playing an instrument, the obvious answer is that your technique is bad. Playing a slower character will make your arm hurt less, but you'd still have that bad technique. Playing an instrument with bad technique is incredibly difficult and requires at least twice the effort to get up to your desired speed, plus it's going to hurt and maybe cause injuries.
You should probably rethink the way you do your inputs and figure out a way to do them that puts less strain onto your arm, and get rid of the bad habits you have. Don't use any muscles you don't have to use, relax, there shouldn't be any unnecessary tensions in your arm. Those tensions aren't going to make you faster at all. You want easy and fluid motions as much as possible. If what you're doing is already difficult at medium-fast speeds, it's going to be insanely hard to impossible if you're trying to go even faster. Figure out a way to make the medium-fast speeds feel like a piece of cake first.
To be honest, I have no idea what it is that you could be doing anyway. Most of what your right hand should do is just moving your thumb and hitting R and Z. That shouldn't put much strain onto your arm at all. Even the Javi claw shouldn't. Maybe your fingers could get tired if you've been playing for hours, but if your arm starts hurting, I think you have to be doing something very wrong. Figure out what it is and stop doing it.
F.e. for wavedashing, there's no need to press L/R insanely hard/fast. A moderate speed will work just as well with the perfect timing. You won't even lose a single frame, just as pressing it even harder/faster won't gain you a single frame. All that matters is that you time it so that you'll wavedash on the first frame possible. Press X, at the same moment start pressing R at the right speed. Don't press X wait 2 frames, then press R insanely hard/fast. It's completely unnecessary. Just as short hopping your wavedashes is.
