C-c-c-combo!
Okay, so whatever Jeb and Dinnerbone did to the lighting engine in 1.5 has absolutely slaughtered my framerate, at least in singleplayer. I mean even on the best days, Minecraft runs like a dog on my system, but I can usually manage around 25-30 FPS, which is at least playable. But the first time I loaded up 1.5, I was getting a whopping 7-10...yeah, try doing anything constructive that way. The only way to get back to something sane was to disable smooth lighting entirely; even setting it to "minimal," which should theoretically produce the same lighting results in previous versions, saw the massive hit. Apparently I'm not alone with the framerate issue, though obviously people who had a lot more to start with won't have nearly as severe of an impact, so your mileage may vary.