I type moderately quickly (at 80-120wpm) but I do NOT type according to what finger should be on what key. INSTEAD I just split the keyboard roughly in half... left to the left and right to the right and many an overlap there is. I also look down all the time (or rather I position the keyboard so I can see my fingers moving on the keys without having to look down).
So it MIGHT just be that you're trying to be too strict with how you're "supposed" to do it, versus what flows naturally. I can't type for scheisse when I'm doing it "correctly" (even though a whole durn YEAR of highschool was spent with paper over the keys so we couldn't look, and we'd be marked down if the teacher caught us using the "wrong" fingers. Seriously. Maybe 15-20 words per minute TOPS.
The other thing that comes to mind is arthritis. Juvenile arthritis can set in quite young, and then just hold fairly steady. If it's not just typing but ALL finger movement is very slow (can't do piano finger exercises - the kind where you tap your fingers against themselves, can't drum your fingers, etc.) you may very well want to go get checked for arthritis. If SO there are a lot of things that can be done which will give your hands more fluidity.
Anyhow... those are just the 2 things that popped into my mind. I suspect there could be a lot of other causes. Not dysgraphia (since typing is the fix for that), but mild dyslexia, a nerve damage, or just the way that your brain operates, etc. I tend to look for "causes" (like mechanics, medical problems, etc.) FIRST... because if there's a cause there's a fix.