How accurately does the P3 Plugin Profiler plugin measure the loading time of plugins, and what mechanism does it uses to estimate the loading time?
Checkout this page from GoDaddy: http://support.godaddy.com/help/article/7360/using-the-wordpress-plugin-performance-profiler-p3
It gives a pretty good overview of the P3 Plugin Profiler and should be able to answer your questions. There's definitely error involved, and it should really just be used as a guideline.
You would have to run the profiling process at least a few times over a couple of days to actually get an accurate result. It isn't just the plugin but other factors such as the varying performance level of your hosting.
For accurate answers, I would recommend triangulating your results by using:
- P3 Plugin Profiler - Google speed test - Yahoo's Yslow
Combining these, or other tools, will help give you a more accurate profile of your website/webpage.
Best of luck.