For the purposes of helping people identify the issue at hand.
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*What the lagging issue looked like for me...*
Starting in late June or early July (around the time of the firmware 1.7 update), I would get spinning beachballs that would last for something like 20-60 seconds, before everything returned to normal as if nothing happened (except the video in Quicktime would have advanced 20-60 seconds when it resumed). These spinning beachballs seemed to be occurring when I was watching a video in Quicktime, copying a few hundred megs from a network drive to my laptop drive, and surfing the web in Safari -- i.e., moderately heavy hard drive use.
*The best way to diagnose the lagging issue...*
Download, install, and run SMART Utility. Take note of the "CRC Error Count" value. When the above symptoms occur, this CRC error count value will increment (for me, it was at a rate of about one count per second -- you may need to tell the program window to refresh for the value to update).
*What has been working for me (until Apple releases a proper fix)...*
If your hard drive has such a capability, limit the speed of your hard drive to 1.5 Gbit/s with a jumper. The CRC error count for my drive had reached 3990, with how frequently I had experienced this issue. After limiting the drive's interface bandwidth, that error count has remained at 3990 (for maybe a week now).
*It should be noted that...*
This reflects only my personal experience with a traditional HDD on a machine with OS X already installed. I can't speak on symptoms or diagnosis methods with an SSD, on problems installing OS X, nor on problems with using/installing other native OSes (bootcamp).