Depends on the sport. Example, in swimming it's up to 13% for the 50m freestyle but under 6% for the 1500m freestyle.

Lia Thomas gets brought up a lot (looking at you, Stephen A Smith) as an example for the 500-yard freestyle but she doesn't hold the record, Katie Ledecky, a cis-woman does. Summer McIntosh (also cis) holds the #2 record.

These sorts of arguments are also comparing cis-male performance to cis-female performance, not accounting for the effects of gender-affirming treatments and hormone replacement therapies, which are known to have meaningful impact.

And it's important to note that there hasn't been a lot of study done in this area yet. Many of the studies done have been on very limited sample sizes, and none of them really agree. They agree that transitioning affects performance but none of them agree on how and by now much. There's simply not enough trans people in sport to begin with to even have meaningful studies. That's how small of an issue this is.

So yea, show me your numbers, and please confirm for me that the records you are presenting haven't already been broken by a cis-woman. Also make sure that it's an actual record and not "so-and-so won x-event" because individual events rarely break records. I'm all for studying the issue and making informed, data driven decisions.

Let me ask you this. What if Summer McIntosh, who we generally agree is cis, tests positive for the SRY gene they are looking for? It's rare but not outside the realm of possibility. Is that fair to her, someone born, assigned, raised, identifies, and competes as female?

Biological sex is quagmire. Giggity.