[O]ne major disappointment with this firmware is that it still uses the RFS file system. The RFS FS is the major culprit behind all the lags and stalling issues which majority of the Galaxy S owners face.
If you want to fix your lag I recommend an Ext4 lagfix. If you flash a kernel such as Voodoo with Odin, you don't even need to root first (note that flashing it will void your warranty anyways though). This guide describes the process for an Android 2.3.4 kernel for the SGS GT-i9000.
I use Voodoo on my SGS Vibrant and it's awesome.