Testing server uptime can be done in theory over perhaps a period of 1 week, every few seconds trying to load the site from various parts of the world. There are services that help you to do this but it can also be arranged by signing up with a few different hosts and writing a script for it, which is definitely a good idea because all the PageSpeed stuff that people normally focus on in technical SEO reports is nothing compared to server speed and failure issues, which the Google PageSpeed tool rightly flags as a priority when it’s an issue.
Factors such as when the site is up and how fast is it in seconds would be different according to locations within the world. Generally a UK-based host is faster for UK and European customers than a US host is. Content delivery networks attempt to solve this but they can go offline themselves and their quality varies between regions. They also dilute asset attribution, for instance a picture coming from cdn.xyz is less useful for the authority building of blaa.com even though blaa.com may own the picture if it is using cdn.xyz to serve pictures and not masking the domain properly.