forum.notebookreview.com a few months back and I'm happy to announce that Dell already has a solution for it. Mr. Todd S. from the http://en.community.dell.com/blogs/direct2dell/archive/2010/02/24/throttling-update-for-studio-xps-1645-customers.aspx">Direct2Dell blogs has posted the solutions.
Symptoms usually occur during gaming or other stressful applications. Can be in the form of extremely large stuttering under certain conditions. Even if these slow downs do not occur it is very likely you could see a performance boost by allowing the laptop to run properly, given a modern game. A quick test is to run on battery, giving the system a few min's to settle, and see if performance is improved.(as per atlstang)
This Studio XPS 1645 throttling issue is a limitation related to the 90-watt adapter. (as per DELL-Todd S)
The resolution is to update the system BIOS, and for some, replacing the 90-watt adapter with a 130-watt adapter.
We released BIOS update A07 for the Studio XPS 1645 system and began shipping 130-watt replacement adapters to affected customers on February 12.(as per DELL-Todd S)
So if you're having some problems playing games or running applications with your STUDIO XPS 16 when your AC adapter is plugged in, you might just be experiencing this same issue.