There have been a lot of posts around the forum regarding lag on the popular Galaxy S i9000, and a few fix ideas have been put forward.

Now, XDA forum member RyanZA has released a beta utility, which aims to fix the lag in just one click.  A lot of people have tested it, and so far reported results are good.

The fix creates an virtual EXT2 filesystem inside the stock RFS filesystem on the internal SD card with a 4kb block size.   This means that this lag fix creates a buffer between the real filesystem and the android system.  This buffer should reduce the amount of disk I/O required for all operations by utilising EXT2 buffering, as well as not writing file access times to disk etc.  It allows only 1GB for application data at this stage, down from the 2GB of application data when running stock.

Currently, if you do not have 1GB of free space on /data, the fix will not run, but RyanZA is looking into workarounds for this.  Requires root.

For full instructions and to download the utility, check out the application thread.