How were the speed rating values determined?

Speed values were often relative to humans and I don't believe they can be acquired through normal gameplay. As you said, back when Nexon provided most of the information, thats probably where a lot of it came from, but nowadays its probably not something worth filling in. If you don't know, just leave it blank rather than try to guess.

For this Ribbon Lightning Dragon 2-Seater, iirc it's functionally identical to another pet (Thunder Dragon 2-Seater). I used values already available.