You can run the Performance Monitor tool.

And then play warcraft, with five clients open.
Also have any background applications that are typically open, opened.
Or if you play with everything closed, then go that route.

After 10-15 minutes of playing.
Check the performance tool, and see which elements have reached 100% or close to it.
Anything that does, is a bottleneck for your performance.
And upgrading those, would improve you play.