You can create a Speed Monitoring check by clicking on the "CREATE NEW MONITOR" button in the Dashboard or at the left-hand-side menu in the Monitoring section.
Choose "Speed Monitoring" as in the first step.
The next step is to enter the website or service name and URL.
It is also possible to get alerted in case your website is taking too long to load.
The last step is to choose who will be alerted once your website or service goes down.