What is GTmetrix?
GTmetrix is a free tool that analyzes your page's speed performance. Using Page Speed and YSlow, GTmetrix generates scores for your pages and offers actionable recommendations on how to fix them.
GTmetrix can also run daily checks (more more frequently with the PRO account) and send alerts if the page load time is above a given number of seconds, or if the grade falls below a threshold. This is the only free web page performance/load time monitoring service available. Similar services (Site24x7, Uptrends) do not have free plans.