Hopefully they'll start with FF4 itself - IME it's a lot slower at eg switching heavy-content tabs. I conjecture it's trading CPU for memory usage, I'd prefer the reverse.
It's also very slow at shutdown and startup. With my profile (including cache) encrypted, restarting the browser takes over 10 minutes. I see the CPU pegged and the Firefox process reading through the entire cache as it shuts down, and again on startup - CPU limited by lsass, the process in Windows that runs EFS operations in user mode, but single threaded.
I had reported feedback to "support" - http://support.mozilla.com/my/questions/802844 - but it seems a bit of a black hole. I've had occasion to measure it more precisely since, it really is on the order of 10 rather than 5 minutes.
It's also very slow at shutdown and startup. With my profile (including cache) encrypted, restarting the browser takes over 10 minutes. I see the CPU pegged and the Firefox process reading through the entire cache as it shuts down, and again on startup - CPU limited by lsass, the process in Windows that runs EFS operations in user mode, but single threaded.