I tried the plugin out.

The only plugin that stood out from the rest was Gravity Forms. Interestingly, I'm also running the Grunion Contact Form plugin on the same site, and it wasn't even shown in the pie graph alongside the giant wedge for Gravity Forms. So I guess if you want optimal performance then use Grunion, not Gravity Forms .... well, assuming the plugin is measuring things accurately, which it may or may not be.