After installing several new plugins I discovered my site is getting slower. Is there a way to know which plugin utilizes the most resource without using the obvious method of activating/deactivating each plugin one by one?
Try this plugin: http://wordpress.org/extend/plugins/p3-profiler/
The P3 (Plugin Performance Profiler) has not been updated in 3 years. It's buggy and doesn't work with PHP 7.
If you use one of the major site performance testers, like GTMetrix, Pingdom or Google Pagespeed, you can use the waterfall feature to get good information, though it's not as easy as using the old P3.
Just go to the waterfall page and slowly move your mouse down over the list of requests. Each one will expand so you can read it as you highlight it. You're looking for requests of the form yoursite.com/wp-content/plugins/plugin-name/xxx. There will probably be a number of these, depending on your site configuration and the page you're testing. Sometimes there will be more than one for the same plugin.
Make a note of these as you find them, along with the timing for that request, which should be a little to the right of the request. If you see more than one for the same plugin, add the times together. When you've completed the list and done any required addition, you'll have a pretty good idea of which plugins are slowing you down.