I never noticed before, but it appears that the fighter capstone's melee and ranged alleged speed increase isn't actually working. It probably never has, but we didn't really have the exact metrics to check and see what stacks and what doesn't.

Doublestrike is 13% - 10% from capstone and 3% from Epic Spare Hand belt.
When I drink a haste pot, melee speeds up to 15%, which is as it should be.
When I have haste boost 4 running, it's 25% melee speed - broken, it should be 30%.
When I have a haste pot and have haste boost 4 running, it's 40% melee - broken again, it should be 45%.
Of course, ranges speed goes up to 22%, so I'm not exactly sure what the fuck is up with that.