This happens since 1.3 (release), probably due to the local worlds system introduced on that update, but I see it hasn't been fixed yet, so I want to report it.
To reproduce:
Just go to any of the menu screens (options, language select, server list, world list) and use, for example, the scrollbar. It looks laggy. Before 1.3, it was fine.
Also, since 1.3, when you select any language, it takes forever to load that language, and that's probably related to this problem.

PD: And no, is not my PC, I have a beefy computer and I can run Minecraft perfectly.