Oh I can run quite fast when I really want to. I've been first in age group a couple of times, on the harder trail courses granted, but still it's amazing the difference 1 day can make when you're changing age group. And yes it's nice to be able to say you were 1st in something, but I didn't get any prizes for it, because there are no prizes. And if I remember rightly the week after when I didn't get 1st in age group, I was actually quicker.
It's just not a race (except against yourself), I enter plenty of races and can categorically tell you parkrun is not a race, your time is never accurate. I press the button on my garmin as I cross the start and again on finish and my "official" time never matches my watch. I don't think that many parkrun courses are exactly 5k either, I've been to ones that have been anything from 2.97 to 3.45miles (that was an epic misplacement of the turn point). The one I do most often is usually around 3.17miles, not a huge amount more than 5k, but still more). It usually takes time to filter through the start, that adds a few seconds, then you're relying on the reaction time of the time keeper at the finish. Plus there's all the other little things that can go wrong.
I run parkrun as a way of seeing my own progress (or lack of), but I don't rely on their timings. I also run parkrun because I like to be able to pick a person in front of me and see if I can catch them up, I don't care if that person is male female or trans.
If I want to actually race then I do an official race, one that has a timing chip on my bib (or shoe), that gives me an accurate time (usually to the millisecond). Parkrun falls more in to the "fun run" category, which BTW I treat the same way that I treat official races because that's how I enjoy running.