Galaxy S hardware is different from previous generation’s Android phones.
To extend /data partition, the place where all your apps are installed and personal data are stored, Samsung use 2GB from the internal SD card.
There’s nothing wrong with this approach, but they use the proprietary RFS filesystem they build as a legacy FAT, adding journalisation and POSIX permission. Both are required to run safely Android.
Unfortunately, this filesystem tries too hard and don’t provide a consistent experience with many applications installed or a few days uptime.
And here comes Voodoo lagfix to the rescue 
By replacing the faulty RFS filesystem with the Linux-standard Ext4 filesystem, Voodoo lagfix simply restores I/O performance and global user experience of the Galaxy S to what it should have been to begin with.
And the performance level is awesome !