I have a Drupal site that is running slowly. What are the best ways to trouble shoot the slowness? Each page load takes ~11 seconds. Once the page has been accessed once it is cached and is fine.
How do you increase performance for anonymous and logged in users?
A) What's the memory you're allowing PHP to use? Look in php.ini and see.
B) How many modules do you have enabled, and how many of them do you really need enabled? Each module you have turned on slows things down a little.
C) Is there a block on every page that's slow? What happens if you turn that block off? Can you optimize this block or replace it with something similar but faster?
D) What are you doing on every page load? Are you using custom modules? Are they doing something in hook_init?
E) Look at the database. Is it slow because you've got too much stuff going on?
F) Have you tried enabling Memcache or something else that helps with caching speed? Can you turn on APC?
G) Do you have a node type with a rediculous number of CCK fields that gets loaded on every page load somehow?
H) Is your theme doing something it shouldn't? Is it custom?
It's hard to say what may be causing your problem, but all of these are things that I'd look into.
Make a copy of your site and install the Devel module. It can display the queries for each page and log them. Most logging options will slow down the site considerably, so you really shouldn't try it on a live site. There you will be able to see if the database is the problem or something else.
You most likely have an unoptimized module that makes too many db queries.
Next would be to use Firebug, Google Pagespeed, YSlow (available as Firefox extensions) to find out optimizations in you resulting HTML, CSS and Javascript.
I would install APC, that caches PHP opcodes and helps with performance. If you have a mostly static site Boost also helps a lot.