Lightroom 4.2 is massively slower than 4.1 in several areas

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Since installing the Lightroom 4.2 update, Lightroom has become very slow for me in several areas:

1. I created a smart collection that has "pick flag is unflagged" as one of its criteria. When I press the x key to change the flag status on an item, in LR4.1 the item immediately disappeared from the collection. It felt almost instantaneous.

Since the upgrade to 4.2 it can take anywhere from a second to 20 seconds for the item to disappear. This makes it impossible to productively work with more than just a few items.

2. Today I imported several 8GB and 4GB cards into my library over the course of a project, and every import seemed to be slower than the last one, up to the point where importing 4GB of pictures took 30 minutes.

Restarting Lightroom seems to speed things up a bit for a while. The issues return though.

I am very frustrated with this drop in performance, everything was good in 4.1 - it pretty much killed my productivity and made me look very unprofessional in front of my client, while we've been staring at the screen, waiting for things to happen.

I would understand if this was a major version jump, but all I see in the release notes are bug fixes and support for new cameras. Nothing that I think warrants such a negative change in performance.

At this point I'd prefer to downgrade back to 4.1 - how do I do that?