We have recently upgraded a few sites to the 9.13.3 version, and have noted the issue of a very slow load time on the first hit of the day (before cache is built up), when trying to open the Users module. The load time is around 2-3 minutes. After the page is loaded once, it usually loads at a normal time for the rest of the day.
Here are a few sites we upgraded today that you can hopefully catch in the act first thing in the morning:
Please let me know if you have any questions.