Casts is between 1.5-4seconds long, what u are implying is that your latency is over 1500ms at every boss cast. Which is either a major overexaggeration or internetproblems on your side.
After one update they did on January for some weird reason my game when i step on SC and Wyrm, which is the same instance/clopypaste zone, my frames go from 50-80 to 8 or 10. If i go some where else my fps go up again in normal numbers - even if i experiment with lower values etc problem persist even if i re-log.
So i would prefer to fix their servers/engine of the game instead of lowering the already low difficulty of the game.