Comment 35 for bug 109943

Err, to clarify what I think you may be focusing on, the pegged cores were very much attributable to TB (as evidenced by Comments #4 and #8, and my personal observations through ps and htop, etc.), not to the the various Window/Desktop managers.

For purely non-TB reasons, I switched to OpenBox: my computer /really/ bogged down after I installed Ubuntu 12.04 (fresh install -- not upgrade -- if it's of use) on my machine last week. (This is still the same hardware as when I started this bug report.) The speedup of Thunderbird was a happy coincidence, and was not an intentional reason for my switch.

TB definitely still has a performance issue, but one that may be exacerbated by the interaction between the more well-known WMs and DMs.