Yeah it's weird, at first the lag (and a later crash) occured when I switched from standard to any mode (actually I just played taiko and ctb but I guess that would happen with mania too). Curiously, selecting a song in the middle of the lag (before the lag grow/crash happens) made osu! go back to normal, and then proceed to play the song properly, without lag or problems, appearing the lag again when going back to the song selection menu. This makes me think it's just related to beatmap processing. Also can confirm this happen with several maps of the bunch, because I tested it deleting only 3 maps (the ones which seemed most game-breaking to me) and the lag was still present.
It may be a hardware problem as well though, my computer isn't that powerful, so that can also explain the bad behavior.
(Not sure if this details would help, but I guess wouldn't hurt to say them)