First time I tested my site, intox.dk, it got an A. Now it gets a D, yet another site of mine (in the same server farm) gets a B.
I like tools like this, but to get more conclusive results, it would need to be able to measure the average loading time (e.g. over a couple of seconds or minutes) instead of just 1 point in time. It even looks like it caches the results for a while, so re-testing immediately does not work.

I have previously used a service called pingfinity.io to monitor my sites like mentioned above and would get detailed statistics of loading time across the continents, but unfortunately that closed down.