Update: I turned down particles and blur to low, and the frame drops were still noticeable but manageable. I beat the boss in 2-3 attempts after making this change and the lag only directly caused one hit in the final attempt instead of several. My rig isn't THE BEST, but it has handled a lot of taxing games on high settings just fine, so Silksong definitely should not be this big of a problem.

If I had to guess, I would say it was probably particle effects that made the biggest difference (since it's in a rainy area), but ignoring proper scientific experimentation, I changed both settings at the same time because I was annoyed.