Sounds like a your running out of RAM, not filesystem lag issues.
Get a task killer, configure it to ignore the apps/widgets you dont want it to kill. Add the task killers "Kill All" widget to your default home screen and hit the kill all widget every time you back out of an application you are done using.
When your captivate is unable to keep more than 100-140MB of RAM free its time for a reboot. If you can never maintain this much free RAM even after a fresh reboot, cut down to 2 or 3 home screens and and less widgets.
With ADW.Launcher, 2 home screens and 3 Widgets total I never fall under 200MB of RAM free (Rooted)
Samsung Galaxy S (Captivate)
ROM: Cognition 2.3b3