Suggestion - just race!
OK I am in my first season of racing so maybe I can address your concerns.
First, your question is a bit of a silly one. There are far too many variables to talk about speeds. How hilly is the course? How long is the race? How technical is the path? In my half dozen races so far, the answer would be wildly different for each. And, I don't even know what the answer is, as it is unimportant. In fact, I took the speedometer off of my bike.
My advice to you is to go ahead and enter a race in the slowest category...and have fun! Start at the back, and while you are waiting to start, talk to the guys next to you. There will be other newbies there too...you will no doubt find lots of support and companionship as you stick your toe in the water.
As for competitiveness, my experience has been that I am astounded how fast people are, even in beginner class. I have finished DFL once or twice, but usually am middle of the pack. One time none of the regular fast guys showed up and I won!
In any case everybody in the beginner class can be schooled by most people in the faster classes, and we're all painfully aware of it. So even while there is competitiveness, there is humility and cooperation and support and friendliness and everybody is just learning...so go ahead and jump in and try it!