Your site's getting popular, but with the increased traffic, it seems slower. You picked a fast host, enabled caching, but for some reason, it's still loading slowly. What's causing the performance issue?
The answer is that it's likely an unoptimized WordPress plugin. One slow plugin can slow down the entire site.
This tutorial will introduce you to P3 (Plugin Performance Profiler). With it, you'll be able to see each plugin's performance on the site.