32-bit programs can’t use as much memory as 64-bit programs. Being able to use more memory means less loading from the disk. 64-bit also tends to be more optimized these days as most systems have moved to it already. In this case, I had no idea TF2 was still 32-bit.
I gave it a Google and saw what changes they have made, and it definitely explains the new feel. Frame timing is waaay better and the frame rate is more stable. They moved from a OpenGL implementation to a Vulkan one, from my understanding. I had to adjust my mouse sensitivity too, so they made some changes there as well.