Thread:Project:Support desk/MW is getting slower with each release

Background: I had been running MW 1.18 and 1.20 (separately, on different wikis) for a long time without major problems, however I encountered time bugs in certain areas of ParserFunctions and also wanted some new functionality that was added for MW 1.22. So to get things done in one fell swoop, I upgraded both wikis to 1.22. Prior to that, I had noticed that the 1.20 wiki was loading a bit slower than the 1.18, but not so much that it presented usability problems.

After upgrading to 1.22, every page now takes a minimum of 5 seconds to load, even simple pages with minimal text. Worse, I randomly get PHP timeouts from a simple search - no way that it should take 30 seconds. And, certain complex pages will consistently timeout now, when they loaded in under 10 seconds in 1.18. I've tried moving my htdocs to a faster drive and that has not helped. The server responds quickly to anything that isn't MediaWiki. Here are samples from my error.log:

[Wed Apr 30 16:43:16.384142 2014] [:error] [pid 1788:tid 780] [client ::1:63172] PHP Fatal error: Maximum execution time of 30 seconds exceeded in [redacted]\\includes\\db\\DatabaseMysql.php on line 76, referer: http://localhost/[redacted] [Wed Apr 30 16:43:16.391143 2014] [:error] [pid 1788:tid 768] [client ::1:63173] PHP Fatal error: Maximum execution time of 30 seconds exceeded in [redacted]\\includes\\GlobalFunctions.php on line 3383, referer: http://localhost/[redacted] [Wed Apr 30 16:43:16.400143 2014] [:error] [pid 1788:tid 760] [client ::1:63174] PHP Fatal error: Maximum execution time of 30 seconds exceeded in [redacted]\\includes\\GlobalFunctions.php on line 3383, referer: http://localhost/[redacted] [Wed Apr 30 16:43:17.462204 2014] [:error] [pid 1788:tid 836] [client ::1:63177] PHP Fatal error: Maximum execution time of 30 seconds exceeded in [redacted]\\includes\\db\\DatabaseMysql.php on line 76, referer: http://localhost/[redacted]

Note that I've also tried rolling back the wiki to 1.18, at which point it is speedy again. So something was definitely changed in 1.19+ that has caused it to suddenly slow down and it appears to be getting worse in each new release. Are there any defaults in LocalSettings.php that I should have changed between 1.18 and 1.22? This feels like either a configuration problem due to changed assumptions, or there's a bug somewhere.

Software versions: MW 1.22.5 PHP 5.5.8 MySQL 5.0.37