Personal anecdote, but I know a few people who got RSI/CTS and had a chance to watch them type --- and what they all had in common was how "stiff" their typing posture was, and the strength at which they hit the keys. They weren't fast either, probably in the 50-70wpm range. One of them, before he got RSI, even commented on how bad my posture was and how it'd "make it easier to injure myself".
I find that relaxing, as counterintuitive as it may sound at first, actually helps with typing speed and comfort. To me it makes sense that if you tense up all your muscles and punch the keys really hard, you're much more likely to injure yourself, slow down, and tire more quickly. I think the vast majority of keyboards also have too high of an actuation force, which also adds to the strain of typing. If you look at videos of fast typists (150wpm+), they don't look like they're exerting much effort at all; many of them aren't using the "standard" key-to-finger mapping, and a lot of them "smear" their hands all over the keyboard. Here's one example (not me):