This is fixed in:
https://github.com/discourse/discourse/commit/e27f3323182ce79d112911b0ae9e5b27c1e7d2d9
This commit has 2 things: 1) the /about page now uses a faster rendering method, and 2) limits the total number of category moderators that are displayed on the /about page to 100. So if there are 4 categories, each one will display up to 25 of its most recently seen moderators.