Zabbix - Track, record, alert and visualize performance and availability of IT resources
GTmetrix - GTmetrix is a free tool that analyzes your page's speed performance. Using PageSpeed and YSlow, GTmetrix generates scores for your pages and offers actionable recommendations on how to fix them.
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
PageSpeed Insights - PageSpeed is addon for ...
Prometheus - An open-source systems monitoring and alerting toolkit.
WebPagetest - Run a free website speed test from multiple locations around the globe using real browsers...