The overall plugin impact on the front end depends on many factors – your theme, the number of plugins installed and the number of redirects you have set up. Here is what happens on a page load for the front end:
When the page starts loading, the plugin uses a hook into init at which time it checks the requested URL against any set up in the Quick Redirects settings (that list is read from the WordPress options table by get_option). If there is a matching URL in the list, it redirects before anything else happens. If nothing is found, it continues on its way.
At the template_redirect action point, it looks to see if the loading post/page has an Individual Redirect set up (using post meta). If it does, it redirects at that point. If not, it continues to load as usual.
There is no Individual Redirect check if the page is an admin page, ajax call, or not a single page (except for the front page itself, if the top level page has a redirect set up).
For Quick Redirects, the init hook is about the earliest we can start checking because the redirects are stored as options and hooking in sooner causes redirects not to happen. For Individual Redirects, the template_redirect hook is the best place to trigger the redirect.
So to sum everything up – If you have a lot of redirects, the check will take more resources before the redirect happens, as there is more data loaded. Additionally, sites with a lot of plugins and/or a resource heavy theme, may also have more resources loaded before a redirect happens, but everything (for both Quick or Individual) happen before any output is sent to the browser. Quick Redirects are more efficient than Individual Redirects but in most cases this all happens so quickly that we are talking about milliseconds.
Hope that helps you out.
Don
