How much does high school senioritis affect transfer applications?

Hey y'all, I'm a high school senior, and I'm currently committed to a college. While I am looking for clubs and labs to join, I'm also considering potential colleges to transfer to, as I have heard some negative comments on the undergrad life there. I've decided to give it my best try in my freshman year, but if I truly find it socially depressing, I don't want to waste four years there. The potential colleges that I'm considering are UPenn and Northwestern; though, I'm worried that my grade in senior year will hurt my chances if I do decide to transfer, because I'm having one or two Bs and some low As. For context, I worked my ass off sophomore and junior year, and was accepted to my top choice through Early Decision, so during the entire second semester of senior year, I just wanted to relax and was in vacation mode.

For y'all who know much more about transferring than I do, do you think a drop in senior year grades will be a major red flag when transferring to T15? Or do they care more about your performance in your freshman year of college?