You asked

great danes top speed is 30 not 40, and the dog can just slow down a few miles per hour

*demon voice* SEND IT

With all the drugs that everyone was on I'm sure that shaggy slipped some PEDs in there

Scooby is a Great Dane?!
X-O
Idk what I thought he was, really
*jumps into the Atchafalaya basin into a gators mouth*

They explained early on that Shaggy was a school track and field star athlete as well. And Scooby probably slowed down a bit.

Usaine bolt isn't being chased by a monster