autoattack is independant from CAs. swinging a weapon with 2 seconds delay attempts to land every 2 seconds. say you're hitting a target with your autoattack and then use a CA with a casting time of 1.5 seconds. it will land and .5 seconds later the next autoattack will hat too.
now use a CA with a casting time of 3 seconds. the CA will hit and right after that the next autoattack that has been sitting in the "queue" for at least 3 seconds by then. after hitting with autoattack it's another 2 seconds for the next autoattack. you spilled 1 second of autoattack with this.
with long delay weapons as bows it is rewarding to find a CA combination that allows for the least spilled autoattack time, resulting in higher dps. not much, but noticable.
that's the game mechanic. i don't think any mod would offer a benefit. just get your CA chains right
