Pc Setup help? Good Benchmark with screen lag?

Now my pc is by far optimal or beast or w.e Very basic set up with

1. Core i3-4160 @3.60 GHz

• 8 GB RAM
• Nvidia Geforce GT 740

I managed to get a 5200 on the benchmark yet during boss mechanics with alot of splahing an what not i move very slow. What can I do to enhance my setup to have a more smooth experience when getting to higher lvl raids/dungeons?