On webpagetest.org, I frequently have a TTFB of nearly 2 seconds. It only occurs on WordPress pages. To test this, I created a simple html document, ran it through the tester, and received an A for the TTFB. I then took that same code, turned it into a page template, and created a page in WordPress—this page gets an F. Something is up with WordPress.
I’ve tried deactivating my plugins, switching themes, etc. Interestingly enough, I also have a slow TTFB on the login page for the WP dashboard.
I’ve contacted my host and they told me it was an issue with my WordPress installation, because other WP sites on the same server are running just fine.
Keith, I wouldn’t suggest “just” disabling it, but also replacing it with Quick Cache. Also, browsing page-to-page, it didn’t feel like the pages were cached. There was a delay going back and forth, so I’m wondering if your cache is even working properly. What version of W3TC are you using anyway as it appears that v0.9.2.6 through v0.9.2.8 have been having some trouble lately due to the number of support requests on the plugin page.
The last stable version appears to have been v0.9.2.5. I never upgraded past that version due to stability concerns.
Sounds like it could have been a database query performance issue. If you have a localhost version you could setup an Xdebug configuration which would allow function and method tracing, and profiling of PHP applications.