I have to be honest.... the Samsung Galaxy S out of the box has a ton of lag - TouchWiz and some other stuff - how it handles external memory is somewhat sluggish.

I have a Samsung Captivate, a Galaxy S variant for AT&T, and use Serendipity 5.12 - soon 6.2 once I get some time. It uses the Voodoo lagfix and I cannot complain. Stock Galaxy S though, yeah... it's sluggish despite having an awesome processor and whatnot.

I don't use Advanced Task Killer... I use Watchdog. It kills processes that are hanging or hogging my CPU. I set the rules to kill anything that kills 30% that aren't phone specific. Works wonders.