If you're clicking and don't hit anything anything(click at air), your ability to click is delayed. Your CPS is essentially capped for a few seconds. This is a vanilla Minecraft mechanic, the NoClickDelay mod removes this.
On 1.8 Minecraft what you're aiming at lags behind what your cursor is over.
You can see it demonstrated here:
https://streamable.com/olr285
That has the game speed dramatically slowed down to better show what's going on, so it's not nearly as bad as it looks there. That delay applies to entities(players) as well as blocks. So in combat, if you're aiming quickly you might not be actually aiming at your target right away despite hovering over them with your cursor.
This is how it looks with the MouseDelayFix mod enabled:
https://streamable.com/my65zr
The game speed is also slowed in that.
Both are presumably bugs and they only exist on certain versions. Neither are detectable in Vape. Some independent mods do the same thing but are banned on some servers. Most legit clients have removed one or both due to it being technically an unfair advantage.