If you want task manager to report as little memory usage as possible you should probably remove LSUseSystemDDE, NetLoadModule, and most label [vars] such as [cpu] as they map DLLs into litestep's address space (in case that concerns you). You could of course just completely remove label. Do the same with all modules using transparency or animations. And remove popup2. Also remove all modules that use system wide hooks as they definitely won't make your system faster.
You could just comment out all modules and then figure out each module's memory usage by adding them one by one.

The "lightest" LiteStep is of course just a blank step.rc (ie. with no modules).