My website is lightning fast (loads under 1 second) using different AJAX and Caching extensions, but all grinds to a complete stop when checking out. Load times can jump up to around 10 seconds, sometimes even more. Even refreshing /checkout/onepage/ can take forever.
I know this issue probably depends on numerous things, but let me begin by asking how I would be able to narrow down the causes. Is there some debugging mode available that can tell me what exactly is being loaded while waiting?