I have a WordPress based website running on a shared hosting. Its response time is very decent (around 2s to retrieve the HTML page and 5s to load all the resources).

I was planning to move it to a dedicated virtual server (Ubuntu 12.04 LTS), which should theoretically improve things and make them more consistent given its not shared. However I observed severe performance degredation, with the page taking 10seconds to be generated.

I ruled out network issues by editing /etc/hosts on the server and mapping the domain to 127.0.0.1. I used the Apache load tester ab to get the HTML, so JS, CSS and images are all excluded. It still took 10 seconds.

I have Zpanel installed on the server which also uses MySQL, and its pages come up quite fast (1.5s) and also phpMyAdmin. Performing some queries on the wordpress database directly through phpMyAdmin returns them quite fast too, with query times in the 10 to 30 millisecond region.

Memory is also sufficient, with only 800Mb being used of the 1Gb physical memory available, so it doesn't seem to be a swap issue either. I have also installed APC to try to improve the PHP performance, but it didn't have any effect.

What else should I look for? What could be causing this degradation in performance? Could it be some kind of I/O issue since I am running on a cloud based virtual server?

I wish to be able to raise the issue with my provider but without showing actual data from some diagnosis I am afraid he will just blame my application.

UPDATE with sar output (every second) when I did an HTTP request:

02:31:29        CPU     %user     %nice   %system   %iowait    %steal     %idle
02:31:30        all      0.00      0.00      0.00      0.00      0.00    100.00
02:31:31        all      2.22      0.00      2.22      0.00      0.00     95.56
02:31:32        all     41.67      0.00      6.25      0.00      2.08     50.00
02:31:33        all     86.36      0.00     13.64      0.00      0.00      0.00
02:31:34        all     75.00      0.00     25.00      0.00      0.00      0.00
02:31:35        all     93.18      0.00      6.82      0.00      0.00      0.00
02:31:36        all     90.70      0.00      9.30      0.00      0.00      0.00
02:31:37        all     71.05      0.00      0.00      0.00      0.00     28.95
02:31:38        all     14.89      0.00     10.64      0.00      2.13     72.34
02:31:39        all      2.56      0.00      0.00      0.00      0.00     97.44
02:31:40        all      0.00      0.00      0.00      0.00      0.00    100.00
02:31:41        all      0.00      0.00      0.00      0.00      0.00    100.00

My suspicion that this comes from I/O related issue is also because a caching plugin I use to reduce the amount of queries to the database, by precompiling PHP pages is actually making things worse instead of better. It seems that file access is making things worse instead.