I am only testing normal attack, not skill(that scale with attack speed).
I am using blunt(mace)/sword/rod.(there is no speed difference)
I am testing on hitting actual target, not swing in air, i am recording the time between the instant first hit damage, and the last hit damage.
Note that the highest Dex i tested is 576.
For every Dex set( in order of 10), i tested twice under different time(morning and night), and to make sure there is not huge difference between them.(two of the same dex set tests within 0.01 HPS(hit per second) )
The result seems like dex affect attack speed like a bracket system, that means, within the same bracket, increase in dex will not get you any attack speed. You will have to invest more to get into next bracket, then, you will see a noticeable difference.
Btw, different skills that scale with attack speed should have their own calculating system,
normal attack 388Dex vs 514Dex has only 8.68% speed difference.
God smash skill with same Dex has 20.35% speed difference.
aim for ~2 HPS(2.05) at 150Dex if you just need decent attack spd(twice per second).
or you can go or ~2.43HPS at 410Dex or 2.51HPS at 490Dex. There is prob no need to go further.
A simple version of graph I made.
btw, the video player i used to get per frame image and the millisecond is MPC-HC.