P3 will only compare your site's performance relative to itself. It should help you detect changes in performance (e.g. when you install a new plugin), but it won't tell you that your site is performing fast or slow.
There are a few reasons for this, but the big two are:
1.) You should decide what fast / slow are. For example, a news site, with mostly static content, high traffic, and a high percentage of mobile visitors will have different requirements than a dynamic BuddyPress site that's used on a company intranet. You can use Google analytics or new relic to get real user metrics (abbreviated RUM) to see how fast your site loads for your users and you can experiment from there.
What these tools will do is say "your site takes 500ms to respond initially" (called time to first byte or TTFB) then will go into the other static resources. P3 will tell where those 500ms are spent in that TTFB metric. E.g. 30% of your TTFB is spent in plugin X, and 25% is spent in plugin Y. There are exceptions, and these numbers won't always line up exactly, but that's roughly the right way to think about it.
I hope this helps answer your question! I'm sorry the answer isn't very straightforward.