I'm also having a very similar issue with the lag and password resetting; I believe the lag may relate to running 1.5.x on the 1.6.x launcher, as something similar happened when I did so in the past when I did so. One fix that might work is going fullscreen, as for whatever reason that ceased any lag on my machine, although we could be lagging for another reason. Our best bet, sadly, would be waiting for all of the mod packs to update to 1.6.x properly. If it still lags, I have no idea what could possibly be going on. :|
(I'm using the newest launcher also, with Mac OSX Lion and the most recent 1.6 and 1.7 versions of Java, as I had to use 1.6 to run any technic mod packs for whatever reason. >_>)