Rebecca Lewis notes the slowdown:
You upgraded to SQL Server 2025. Your SQL auth logins are slower. This is not a bug. Microsoft did this on purpose — which is actually good news, but if you’re not using connection pooling, it’s not going to feel very good.
Read on for more details, as well as a recommendation for connection pooling. On the whole, this is a very good thing—PBKDF2 is a mature process that we’ve had available to use in applications for 15+ years, and what’s particularly neat about it is that the level of effort scales linearly. As computer hardware becomes more powerful, add more iterations of the hashing algorithm to keep a fairly consistent per-login delay.