Here's a fix by Tayutama at xda-developers.com. I've edited the instructions to be more accurate after trying it out myself.
1. The update.zip installs all the files needed. It is attached here; http://forum.xda-developers.com/attachment.php?attachmentid=377244&d=1281437129
2. Place it in the top folder on your internal sdcard, ie. /sdcard
3. Turn off your phone.
4. Hold down Up(Volume) + Power Button + Menu button(Home button) simultaneously for a couple of seconds until u see the manual boot screen.
5. Use the up and down buttons to get to update.zip.
6. Press menu button on update.zip to select it
7. It installs root, correct busybox (1.17.1), superuser.app, as well as all the scripts needed for the fix and the 'MCR Scripts' app to run the scripts.
8. After install, the phone restarts.
9. Go to the newly installed 'MCR Scripts' application from the app list, open it and choose from 3 scripts. For example, Lagfix10024 is the 1gb ex2 version.
10. The script would launch superuser permission prompt. Click "Allow" and it should run in a terminal for about 10 minutes. Don't worry if it seems to get stuck on "...takes a while...". You'll see some output of what it's doing later.
11. You should see an 'All complete' terminal prompt, and this means you can close the application.
12. Run Quadrant app (install from Market) to test if the fix was applied. You should see results like in http://www.droiddog.com/android-blog/2010/08/galaxy-s-lag-fix-makes-phones-fly/
**PLEASE READ!!! Some hiccups that I've encountered.**
The first time I clicked "Allow" in step 10, the script encountered Permission denied and didn't do anything. I had to stop the script and try it another time before it got going.
Before 'All complete' prompt, the script seemed to have repeated what it needed to do. After "installing" my Market app couldn't work. Rebooting phone caused screen to be blank. I had to press Up + Menu + Power to enter manual boot and do Factory reset. That meant losing data on my phone. At least I didn't have anything important irrecoverable as my Contacts are synced with Gmail Contacts. I installed and ran Quadrant just to see if the fix had been applied and saw that benchmark results were that of before fix.
I tried again by repeating from step 3. This time the script seemed to have executed properly. I ran Quadrant again and got the benchmark results. Running applications did seem MUCH FASTER but I hope it's not because I've factory reset the phone...