What yawnzz said is half wrong half right.
thing is, ASPD plays a role in almost every skill and it becomes more notable the more -aftercast gear you get (or bragi)
imagine having -100% aftercast delay (3 kiel and bragi, or something) .. but you only have 160 ASPD. This means the maximum you can ever spam, regardless of the fact that you have -100% aftercast delay, is 1.25 desperados per second.
Less ASPD with -100% aftercast means you're capping yourself, while, for example if you have 190 ASPD you could be doing 5.00 desperados per second.
You'll have to find a balance between ASPD and -aftercast.
ps:
If you have NO -aftercast gear at all, the difference between 160 aspd and 190 aspd is NOTHING when it comes to spamming
desperado.
edit:
so yes, it's a different factor.. and there are like 4 in total:
ASPD
Client sided animation delay
Aftercast
Cast time (invisible(bowlingbash) or visible)