I noticed that Google Chrome on Mac became occasionally slow and unresponsive over time for me. This has happened both with Chrome 5 and Chrome 6, and generally took several weeks to develop.
Browser startup and opening of new tabs were particularly bad, sometimes causing the browser to become unresponsive for up to 10 seconds.
Disable DNS prefetching and clearing the browser cache seemed to help a little, but what eliminated the problem was to turn off the 'recently closed tabs' option on the new tab page (with the thumbnails of frequently used sites). The option is on the top right of this page.
I hope this is of use to someone else.