Plugin allow you to run most important test for performance: request time, number of database queries, memory usage
You will be able to detect plugin which cause troubles and result with longer waiting time for request results
Testings are executed based on your homepage
How to use it?
In Dashboard go to Tools->Profiler What Slowing Down
Click on run test on all plugins (first link) and then near wanted plugin, run few tests and compare results
If you get significantly lower results, means that this plugin causing slowness
Hint: Lower numbers are better, means that page load faster without this specific plugin