It is very simple. For our template sites we guarantee that your web site will consistently score at least 90/100 on both mobile and desktop in Google PageSpeed and will load in less than 2 seconds as measured by GTMetrix. Or we will credit you for an entire month's service!
Because there are variables completely outside of our control the only caveats are these:
The reason for this is, in the case of Google PageSpeed, it is possible that there was a "bad load" and Google is reporting on that. The multiple test runs should correct for that. It is also possible that Google changed their testing algorithm and we have not yet had the opportunity to update the template.
In the case of GTMetrix (loading time), there are many variables. There could be Internet congestion outside of our servers that we have absolutely no control over. There could be an issue with GTMetrix. Etc.
We are fair. What we are trying to guarantee is that our code and our tech will perform consistently. Please understand that we cannot be expected to guarantee performance outside of our own code and tech and we will certainly work with you to make sure that you get the highest possible performance!