I read project readme, checked client side and server side configurations, and no such feature exists - YES

Is your feature request related to a problem? Please describe.
I wanted to make the first 2 rapier attacks slow and the next 3 fast.

Describe the solution you'd like
Add the ability to change the speed of each attack the same way it is done with damage. Without this feature, creating a unique and interesting attack pattern for a weapon is much more difficult.

I agree it would be nice to have such feature.

I have been thinking about this feature, and unfortunately I came to the conclusion that there is no good way to implement it.

Faster attacks within a combo would deal less damage due to the weapon cooldown reducing it. This could be automatically counteracted.
However in such scenario it would be easy to just wait before doing the quick attack, deal full damage, with the added bonus damage of speed counteract. Leading to exploitable combat.

This feature requires a great deal of math magic to get it work.
Anyone with the capacity to dwell into this is welcome to PR it.

Can't you just use the formula damage + (base_damage - damage)? In this case, it doesn't matter how charged the attack is, the damage will always be the same.

There is no such variable as base damage.
Hence no.