Maybe some small calculations will help.
2.0 speed
100 damage per swing
60 seconds
(60/2)*100=
3000 damage
25% faster
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2.0/1.25= 1.6 (-0.4)
(60/(2/1.25))*100= 3750
750 damage gain
% of original value: 750/3000= 25%
again 25% faster
----------------
2.0/1.5= 1.33 (-0.27)
(60/(2/1.5))*100= 4500
4500-3750= 750
750 damage gain
another 25% faster
------------------
2.0/1.75= 1.14 (-0.19)
(60/(2/1.75))*100= 5250
5250-4500=
750 damage gain
So it doesn't matter how fast your weapon already is, X% more haste will always net you X% more damage
of the original value.