Is there any way to measure the time it takes for plugins to load and in which pages they load (without figuring out the entire code)? In addition, I would like to find out which plugins load files in which pages and find the ones that load on irrelevant ones.
Yes, Plugins effect site loading time.
There are many poorly written Plugins that make unnecessary database queries and load numerous files to the page.
To see what extra files are being loaded you can simply view source or use the net tab on Firebug.
Another good way to find external files loaded by plugins is to use W3 Total Cache's minify help wizard.
The wizard scans your theme and plugins and recommends the ones to combine and minify and for what page to load them on.
To monitor Plugin database queries and cpu time you can add the following code to the bottom of your footer.php:
<!--<?php echo $wpdb->num_queries; _e('queries'); ?>. timer_stop(1); _e('seconds'); ?>.-->
This will allow you to view the source of each page and see how many queries are being made and how long it takes to make them.
Of course this won't tell you what Plugin is responsible for the queries but allows you to do testing while disabling and activating different combinations.Tweet