I tried a lot of fixes, some expensive:
1) Install new RAM, taking it from 1gb to 4gb
2) Install a larger hard disk drive, taking it from 120gb to 320gb (now using the 120 on a Linux machine and it reports as being perfectly healthy, as it did within OSX)
3) Added the entire HDD to the "privacy" list in Spotlight to prevent indexing of the entire disk
4) Disabled indexing from the command line (and did not turn it back on)
5) Ran Disk Utility | Repair Disk Permissions
All of those are things that various posters in various forums suggested as a fix to what sounded like the same or very similar problems, but none of them worked. Finally I had an idea to search on terms related to the login screen and fast user switching rather than things like "slow," "unresponsive," and the like. Something I saw caught my eye relating to fast user switching and a difference in the way the Accounts utility presents the options for fast user switching between newer versions of OSX and older ones. Most interesting is the fact that we were not using fast user switching; if I was logged in and my wife wanted to use the computer, I had to log out and then she could log in. I think at one point I may intentionally have disabled fast user switching, thinking that it was a way to conserve system resources and therefore improve performance.
It turns out that with Snow Leopard you don't so much get to choose between *whether* to use fast user switching and single-user-only mode, you just get to choose *how* OSX goes about it by choosing an option in the "Show fast user switching menu as:" dropdown list, which has a checkbox next to it that controls whether you actually use fast user switching. On our machine that option was unchecked, so I checked it and left the default option of "Name" in the list. Here's what it looks like in the Accounts screen:
The difference is like night and day: no more lag, no more wondering if the screen ever will come back, it just works. So far I have tested by locking my screen, logging in as a different user, locking that screen, switching back to my screen, and other tests like that. I also have logged completely off a user and back on as an already logged-on user, and a couple of other combinations that fit the pattern of our normal, regular usage of the machine, and they all still work. What I haven't tried yet (because I wanted to post this blog entry and not risk being unable to log back in immediately) is powering the computer all the way down and turning it back on, which also exhibited the same frustrating behavior.
At the moment I'm not feeling brave enough to test the negative by unchecking the box to see if the behavior returns, but that's my hypothesis. Looking at things from a black box perspective, it would seem that there is some kind of hangup within OSX where it lags like crazy when switching users if you are in Snow Leopard and if you have unchecked the "Show fast user switching menu as:" checkbox. Rather than letting the user check the checkbox that causes all the trouble and does not otherwise seem to control any option, perhaps that checkbox should just go away.
If you are having the horribly frustrating behavior like we were having and have tried everything else to no avail, take a look at your Accounts settings and see if the fast user switching checkbox is unchecked; it's about as simple a solution as you can get to an incredibly frustrating problem, it's just not at all obvious or even intuitive how to go about finding the fix.
