Monitor Drupal / PHP Code?

What tools and resources are there for monitoring a Drupal site, regarding page load times, browser compatibility or bandwidth limitation, and overall health?

And the tools you recommend, are they specific to Drupal, or to PHP applications overall?

I realize this is broad. Part of this is resolving why our development environment is slow.
sandshakimiAsked:
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Ray PaseurCommented:
Google Dev Tools is your friend.
https://developer.chrome.com/devtools
http://discover-devtools.codeschool.com/

There is also YSlow.  You can profile your SQL queries with EXPLAIN SELECT.  

In my experience, "slow" almost always means the problem exists in the I/O subsystem and in modern web applications that means the data base, and sometimes also the file system.  This can be particularly true if you have auto-loaders that repeatedly request parts of the application each time a page is loaded.  Auto-loaders are a friend to developers who want an easy way to do a build, but they are out-of-place in a deployed application.  A proliferation of CSS and JS files can cause a similar effect.
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