I have run into this issue when I was on horrible ISP.
1. Warrior was a struggle - Fell Cleave spam
2. Monk was a struggle - Perfect Balance spam
3. Paladin was a struggle - Holy Spirit + Confiter
4. Machinist was a struggle - Heat blast Spam
5. Summoner was a nightmare - Dreadworm + Deathflare, Wyrmwave + Ahk Morn, and the phoenix burst phase respectively.

You start to see the pattern here if you ping is really high.

Once I was able to switch to an ISP with very good bandwidth and very stable ping of around 30-50 everything above literally became non-existent.

I found it strange that SE decided to design the game this way when you see other MMO's with an incredibly faster GCD (WoW) and not really suffer from the input lag. Maybe it's due to SE's servers architecture or net code of the platforms being used. I don't know.

But at the end of the day. Faster internet makes this game MUCH more enjoyable when your trying to min max a rotation.