I always use the Simple Testing tab when performing tests on WebPageTest. In order to get an A in First Byte Time, the web page needs to produce a value of ~1750 ms or lower. This is I think unattainable.
I’ve tested on a bunch of very fast websites, and the fastest web page I found got a C with a time of 2049 ms 1. My own in-development website got an F with the time 2158 ms. I wouldn’t be surprised if 99.9% of websites get an F in Simple Testing mode.
Should this grading be adjusted? It’s not a very useful grade if all websites with a few exceptions get an F. It can even be discouraging. I suggests that ~1750 ms is raised in Simple Testing mode to a more attainable value that can realistically serve as a target for websites.
Edit: I’d like to clarify that when I say “Simple Testing mode,” I mean the default configuration, which includes the “Slow 3G” setting. I’ve tested with “Regular 3G,” and it is possible to get an A grade with it, so it’s really just the default, “Slow 3G,” that is causing the grading issue.