I agree IPB 3.4.5 which I recently shifted from my old SMF 2.0 forum is comparatively slow. Yes, SMF was not having these many features as that of IPB but it used to load amazingly faster like a basic html file. For now I am using maxCDN with my IPB install to faster some aspect but still not upto the expectation. Xenforo on the other end is faster than IPB.
Note: I am discussing about the load time and not any features.