P3 Plugin Profiler Says WP Rocket Uses The Most Resources

P3 Plugin Profiler can be a useful tool in diagnosing slow websites, but it's also easy to misunderstand. P3 Profiler attempts to measure how much of your total page loading time can be attributed to each plugin. But when using a caching plugin, P3 provides very misleading results. When you use a cache plugin, it loads before everything else and will appear to take up all the resources, but in fact it's because it's replacing all the normal processes that WordPress would typically have to run through when loading your site. 

How does a caching plugin like WP Rocket work?

WP Rocket takes all the content of a given webpage and stores it as a single, static HTML file which is displayed to your visitors. Serving a static HTML file is much faster than the normal loading of a WordPress page which involves executing PHP, making calls to the database etc.  

So, since WP Rocket is replacing all those other functions, P3 reports that it is using the most resources. It doesn't show that this is what is making your site FASTER, not slower!

Cache generation

There may be a spike in activity from WP Rocket if a cached version of a page does not yet exist, because it has to process the page and its content in order to generate the cache file. But once that is done, it's very fast to serve the cached version after that. 

Load Time Is The Real Indicator

So P3 is not a real way to test the effectiveness of WP Rocket. For that, you need to  speed test the front end of your site using Pingdom Tools following the directions provided here:
How To Correctly Measure Your Website’s Page Load Time
The point of a caching plugin is to make your site faster. So all you need to do is disable WP Rocket, speed test your site, then enable WP Rocket and speed test again. You'll see your site is much faster with WP Rocket therefore it doesn't matter if P3 provides a misleading result.