When logging in and visiting only particular pages as administrator,
Moodle 2.5.1+ (Build: 20130719) is very slow - 10-20 seconds or more. Fresh install, there are no other users on this GNU/Linux site.

Here I am revisiting the front page, where lists, blocks etc are all turned off, i.e. - just the front page. About 10 seconds to load this time:

9.970462 secs RAM: 57Mb RAM peak: 57.1Mb Included 661 files Contexts for which filters were loaded: 1 Filters created: 0 Pieces of content filtered: 0 Strings filtered: 0 get_string calls: 951 DB reads/writes: 37/2 ticks: 997 user: 124 sys: 53 cuser: 0 csys: 0 Session: 7.2Kb Caches used (hits/misses/sets)core/string** static persist *: 783 / 484 / 0cachestore_file: 484 / 0 / 0core/pluginlist* static persist *: 3 / 103 / 0cachestore_file: 62 / 41 / 41core/plugintypes* static persist *: 896 / 1 / 0cachestore_file: 0 / 1 / 3core/yuimodulescachestore_file: 1 / 0 / 0core/repositories* static persist *: 0 / 9 / 0cachestore_static: 0 / 9 / 18core/plugininfo_base* static persist *: 4 / 883 / 0cachestore_file: 881 / 2 / 317core/plugininfo_mod* static persist *: 72 / 1 / 0cachestore_file: 0 / 1 / 1core/plugininfo_block* static persist *: 46 / 1 / 0cachestore_file: 0 / 1 / 1core/plugininfo_filter* static persist *: 5 / 1 / 0cachestore_file: 0 / 1 / 1core/plugininfo_repository* static persist **: 19 / 1 / 0cachestore_file: 0 / 1 / 1Total: 3256 / 1541 / 383
This page is: General type: frontpage. Context Hovedside (context id 2). Page type site-index.

Here I am visiting the Messages page. About 10 seconds:

10.708157 secs RAM: 67.4Mb RAM peak: 68Mb Included 685 files Contexts for which filters were loaded: 1 Filters created: 2 Pieces of content filtered: 1 Strings filtered: 0 get_string calls: 2065 DB reads/writes: 88/2 ticks: 1071 user: 130 sys: 55 cuser: 0 csys: 0 Session: 5.2Kb Caches used (hits/misses/sets)core/string** static persist *: 1867 / 516 / 0cachestore_file: 516 / 0 / 0core/pluginlist* static persist *: 5 / 104 / 0cachestore_file: 63 / 41 / 41core/plugintypes* static persist *: 1338 / 1 / 0cachestore_file: 0 / 1 / 3core/databasemeta* static persist *: 0 / 1 / 0cachestore_file: 1 / 0 / 0core/plugininfo_base* static persist *: 4 / 883 / 0cachestore_file: 881 / 2 / 317core/plugininfo_mod* static persist *: 72 / 1 / 0cachestore_file: 0 / 1 / 1core/plugininfo_block* static persist *: 46 / 1 / 0cachestore_file: 0 / 1 / 1core/plugininfo_filter* static persist *: 5 / 1 / 0cachestore_file: 0 / 1 / 1core/coursecatcachestore_session: 0 / 1 / 3static session: 0 / 2 / 2core/coursecatrecords* static persist *: 0 / 3 / 0cachestore_static: 0 / 3 / 3core/plugininfo_repository* static persist **: 19 / 1 / 0cachestore_file: 0 / 1 / 1core/yuimodulescachestore_file: 1 / 0 / 0Total: 4818 / 1566 / 373
This page is: General type: admin. Context Globalt nivå (context id 1). Page type admin-index.

Here I am visiting the modules/plugins overview page. About 1 second:

1.127477 secs RAM: 66.2Mb RAM peak: 70.8Mb Included 366 files Contexts for which filters were loaded: 1 Filters created: 2 Pieces of content filtered: 1 Strings filtered: 0 get_string calls: 3148 DB reads/writes: 73/2 ticks: 112 user: 89 sys: 23 cuser: 0 csys: 0 Session: 5.9Kb Caches used (hits/misses/sets)core/string** static persist *: 2925 / 595 / 0cachestore_file: 595 / 0 / 0core/pluginlist* static persist *: 3 / 79 / 0cachestore_file: 79 / 0 / 0core/plugintypes* static persist *: 3489 / 3 / 0cachestore_file: 3 / 0 / 0core/databasemeta* static persist *: 0 / 1 / 0cachestore_file: 1 / 0 / 0core/plugininfo_base* static persist *: 1201 / 2 / 0cachestore_file: 2 / 0 / 0core/plugininfo_mod* static persist *: 166 / 1 / 0cachestore_file: 1 / 0 / 0core/plugininfo_block* static persist *: 209 / 1 / 0cachestore_file: 1 / 0 / 0core/plugininfo_filter* static persist *: 34 / 1 / 0cachestore_file: 1 / 0 / 0core/plugininfo_repository* static persist *: 79 / 1 / 0cachestore_file: 1 / 0 / 0core/yuimodulescachestore_file: 1 / 0 / 0core/plugininfo_portfolio* static persist *: 0 / 12 / 0cachestore_file: 11 / 1 / 1core/coursecatcachestore_session: 1 / 0 / 3static session: 1 / 1 / 2core/eventinvalidation* static persist *: 0 / 2 / 0core/coursecatrecords* static persist **: 0 / 3 / 0cachestore_static: 0 / 3 / 3Total: 8804 / 706 / 9
This page is: General type: admin. Context Globalt nivå (context id 1). Page type admin-plugins.

I do not have PHP acceleration available, which I initially thought was the problem. However, this does not explain why the front page is fast for any other user, like guest access, and it does not explain why only a few pages are affected.

The behaviour is consistent with what is discussed in this forum post - "Performance hit on 2.5 upgrade for admin users" by John Porten -

https://moodle.org/mod/forum/discuss.php?d=231040

I wonder if some of the other discussions about 2.5 poor performance might be attributed to this one, but I cannot confirm this, just a hunch. After all, the administrator is the first one on the site, checking things out.

Thank for looking into this issue as soon as possible!