Been over this already in several forums. Well known issue that doesn't seem to affect everyone.
OS 10.6.7, 2 year old iMac with 8gb Ram, 2.66ghz/2 core. Did an erase and reinstall of the HD at the time of updating to LR3. I work with pretty large RAW and Tiff files (Canon 5dII).
I have reported to Adobe via their bugs reporting process. No help.