Just wondering if anyone else has had their entire Minecraft messed up since the 1.8.1 launcher was released? My minecraft ran perfectly, I was using a modded 1.7.2 version and had no issues. All of a sudden any version lower than 1.8 is buggered and lags like a mutha. I can't even play it. So I have done everything - updated java, downgraded java, installed the new launcher, taken out the mods - aaarghhh! I am pissed off because we had a world that we had so much work done on and now we can't bloody use it.
The new launcher defaults to its own special version of Java 8; you have to edit your profile to use a different version, as I have for the very same reason, even causing possible world corruption (I found a chunk that regenerated; luckily it was in the middle of nowhere and I had a backup I used to restore it). In fact, I avoided updating the launcher (I had downloaded Java 8 u25 separately beforehand) until the launcher started pestering me about outdated Java and all that (I was dumb enough to try using the "special" version of Java included with the launcher, which is when the regenerated chunk occurred (I also saw occasional errors about item entities in the logs both times when I used Java 8, possibly connected to the regenerated chunk, though they occurred far away as well), as they claimed it was the best possible Java for your system; no difference from regular Java 8 that I could see):
(in my case I'm running a self-modded (non-Forge) version of 1.6.4; note also that there is a serious compatibility issue (crash on startup, I don't think this does anything about the lag) with Forge and Java 8, unless you install a patch, only included with 1.7.10 and later in the official Forge downloads)
The new launcher is Java 8 (as said above). This was added because lots of people who were using 1.8 had complaints about, and the reason for that as because they used Java 7 or Java 6. If your lagging on any version other than 1.8.x I recommend getting Java 7.