Usually 10% is a flat +1 aspd more or less.
How much % gives depends a lot on your agi. With low agi, % increases are incredible.
The aspd that you gain from a given +% effect depends on only three things:
* Your current aspd without the modifier, before accounting for any +aspd effects.
* The sum of all other +% aspd effects that stack additively
* Whether you're third job or not (third job can get 193 aspd, otherwise, limited to 190).
200-old aspd = old swing delay
Swing delay w/+% aspd effect = (old swing delay)/(1-(sum of +%aspd effects, not including this one))*(1-(sum of +%aspd effects including this one))
Change in aspd = Old swing delay - Swing delay w/+% aspd
a bit of algebra and:
change in aspd = (200-old aspd)*((1-sum of % aspd mods)/(1-sum of % aspd mods, incl new mod))