Someone who can complete a 5km park run in 30 minutes has a time of 30 minutes no matter what.
If you are of the male sex and have a time of 30 minutes you are around about average, perfectly respectable time but nothing special.
If you are of the female sex and have a time of 30 minutes you are a fair bit above average, and that's a higher level of personal kudos
If you are of the male sex and have a time of around 20 minutes you are certainly very good but depending on how popular the run is there could be about 10-20% of the other males in the field able to run faster. Pretty impressive personal kudos.
If you are of the female sex and have a time of around 20 minutes you may well be the winner. Maximum kudos.
Every time a male identifies into the female category they are not only claiming kudos they haven't earned, they are also demoting and degrading the results of every other woman in the female category by skewing the stats, so someone who previously knew they could run faster than 80% of runners in their category now only gets 79.something% based on the false data. Each individual time it happens only makes a tiny difference but each one is cumulative and devalues the whole.
Park runs are also split by age - the average winning time for 20-29 year old men is 16:45 minutes but the average winning time for 60-69 year old men is 22:36 minutes. If a man age 25 can run the distance in 23 minutes they are obviously fast and fit but not amazing. If he chooses to identify into the 60-69 year old category that time becomes amazing. Allowing him to do that would obviously be wrong and unthinkable and no one would let it stand. But it's the exact same effect as what we are just expected to accept for women because our sense of achievement doesn't matter as much as the feelings of transwomen.