You fixed it!
I modded my Vostro 220 a few years ago and had the same problem: very laggy windows 10 system due to high DPC and ISR CPU usage. Flashing your modded bios based on 1.2.0 immediately solved the problem.
I didn't need to disable Speedstep or C1E support. I only disabled C-STATE.
Thank you for sharing!
Additional info:
For the last 2 years I had Apple OS X installed on my Vostro. Strangely enough, the C-STATE incompatibility didn't effect OS X. It only caused a problem for Windows 10.