Using 4.2 and have not noticed any speed issues. I don't have a lightning fast machine, an AMD quad core with 4Gb RAM which is 2 years old now, but LR4 seems to run just as fast as 3.4 did.
For a quick dirty fix on RAW files the auto mode is much better with the 2012 profile. The 2010 profile used to overexpose shots quite badly but the 2012 makes more effort to preserve the original lighting.
It's a bit bloated, yes, but useful for me. I can process all my RAW files and export them, usually to Jpeg for the web, at full quality easily and quickly. The geotagging feature is useful as is the Gallery, which I am still playing with.
It still fills your hard disk up with copies of everything but in these days of huge cheap disk space I can live with that. It is also very easy to use for my disabled partner, who picked up the basics in a few minutes, something not many people would think about.