I disabled W3TC for now so I could see the different in load times. I can go ahead and enable it again if you think you can help with it.
1. I did try disabling all other plugins, but unfortunately, it didn't help load times at all.
2. PHP cache is already enabled. I do not see Jabber in that panel in particular. If I go to "Jabber IM" under the Goodies section, there are no users, so I assume it's disabled?
3. I tried Pagespeed until last week when the last DH tech I spoke with told me to disable it. I didn't see a difference in load times one way or the other anyway. (I confirmed with Google that it was in fact set up properly, so I know that wasn't the issue.)
4. Are there any risks in switching to PHP 5.4? If it breaks anything, will I be able to go back to 5.3 and have everything work again?