Good Performance is very important for SEO and Onpage Optimizing! If your Site loading longer than 20 seconds then you maybe have rankingproblems.
I always check my performance at: GTmetrix | Website Speed and Performance Optimization
My Pageloading time was 23 seconds... I reduced it with some wordpress settings to 4 seconds...
When I install a new plugin.. I always check the performance after this.
At "history" you can compare the performance before and after installing and using the new plugin.
For example I installed a flashgallery from jquery... and checked the performance and I had 20seconds loading time.
When I was delete this plugin I can see in the History that my perfomance was much less than before.