framespeed=1 - eight times faster than the original frame speed.
framespeed=2 - four times faster than the original speed.
...
framespeed=4 - two times faster than the original speed.
...
framespeed=8 - normal. No changes to the framespeed.
...
framespeed=16 - half as fast as the original framespeed.
So, if you're good at maths, you can see that the framespeed= values are eights of the original frame frequencies. 1 makes it 8 times faster, 8 does nothing and 16 makes it twice as slow. So, in percentage, the values would go like this:
framespeed=1 - 800%
framespeed=2 - 400%
framespeed=3 - 267% (rounded)
framespeed=4 - 200%
framespeed=5 - 160%
framespeed=6 - 133% (rounded)
framespeed=7 - 114% (rounded)
framespeed=8 - 100%
framespeed=9 - 89% (rounded)
framespeed=10 - 80%
framespeed=11 - 73% (rounded)
framespeed=12 - 67% (rounded)
framespeed=13 - 62% (rounded)
framespeed=14 - 57% (rounded)
framespeed=15 - 53% (rounded)
framespeed=16 - 50%