I, too, have recently upgraded from 1.6.x to 1.11.0 and as such, I also had to switch from PHP4 to PHP5. And, I too, have noticed an unacceptable delay in getting pages delivered. (I had though it was a problem with my browser's rendering speed, but I now know this is not the case.)
The earlier system also had severe delays and my host (GoDaddy) turned off the Zend Optimizer. It got real quick then. Really - turning off the optimizer made it quicker.
However, with PHP5, there is no optimizer in play and I've not finished my analysis of the available extensions available.
Personally, I'm leaning towards delays in accessing and receiving recordsets from the database as the cause.
I have a developement installation on a local machine as well. No delays there.
GoDaddy (Linux Apache) * MediaWiki: 1.11.0 * PHP: 5.1.4 (cgi-fcgi) * MySQL: 4.0.27-max-log
Local (Windows 2003 Server SP2) * MediaWiki: 1.11.0 * PHP: 5.2.4 (isapi) * MySQL: 5.0.45-community-nt
I have succesfully moved my wiki site to another location and upgraded from 1.6.5 to 1.11.0. The new site works well, except for the speed in generating the pages.