Thanks for the tip. After deleting mine, I freed some 10-12 GB of disk space, but the improvement in the LR3 performance appears to be rather marginal. The memory usage still creeps up to almost 2 GB after a while. The most annoying thing is working with adjustment brushes and having to wait for long seconds until you can actually see the effect... very often, by the time you see it, you've overdone it And I am currently working with a catalogue of merely 5,000 images.
I paid more attention to your gear.. An Imac is merely a laptop processor/system in an all in one.. so I played with a couple of my laptops and catalogs of some 20,000 images.
My main workstation is an i7 running at 3.8g with 12gs of RAM, a SSD system drive, and my indexes/previews on their own drive. There is no delay with the adjustment brushes unless I build up more than 12-15 adjustment points and from there it slows down. There is also, and always has been, a 1-2 second delay before the changes show up on the second monitor. This has always been irritating and I hope they improve it some day. This system has 164,xxx images in its catalog.
My Dell Precision Mobile Workstation with 4gs of RAM, a Core 2 Duo 2.5g, 1g quatro video, mechanical hard disk.. It basically performs as you describe with a catalog of 10,000 images.
My new Lenova x201s 2.5 pound laptop.. not really a processing machine but it is fast. i7, 4gigs of RAM, SSD, and scores significantly better than the Dell above on the Windows Index.. It works great.. up to the point of 6-7 adjustment points.. and then it starts to slow down.
I also tried 2.7 and didn't notice much change from 3.0.
It appears there is a hardware factor when using LR. We've always known this but we've never talked about it much.. Since I do most of my work, and virtually all of my heavy work, on my main workstation it doesn't affect me much. But the few times I was forced to use my Precision Mobile Workstation in the field I sure did.. and an Imac mechanically is pretty much the same.. unless you have one of the brand new ones..