In my opinion it is all about the actual course for that particular race.
I had a bit of trouble getting to MK the morning of the Princess Half back in Feb. The route for that race started at Epcot, went to MK, then back. I was trying to get there for when they open Main Street 7:45-8 (9am opening), and the trip from BWI to MK took quite a bit of time. According to my Minnie Van ride history, we left BWI at 7:08am and arrived at CR at 7:42am. That’s 33 minutes for 7.2 miles. Not the end of the world, but not fast either. And don’t even ask why I had to be dropped off at CR.
Once in the MK, runners were still heading in/up Main Street until approx 8:15/20ish. Once they were through the route and people instantly disappeared and it was like any other day at the MK, but I think the travel delays around World Drive continued longer.
Anyway, point being... pay attention to the course of the race once they publish it and be prepared to add some extra time to your travels if the course impacts your desired park or transportation route.