Geforce 820m slower than intel integrated graphics (bay trail)

[domovoi]

It's really quite bizarre. When I force PPSSPP to use integrated graphics, I get pretty good, though not perfect, performance for a game like Tactics Ogre (usually 100%, but the occasional framerate drop). But when I force it to use the 820m, performance drops significantly to around 40-60%.

Using MSI Afterburner, I can actually see that the GPU load for the 820m is 100% when I tell PPSSPP to use the 820m, and 0% when I tell PPSSPP to use integrated graphics, which is what I expect. It might be a driver thing (though I've tried both the ASUS-provided driver and the most up-to-date Nvidia driver using a modified INF file), because I also see a performance discrepancy using Dolphin (though to a much lesser extent). But when I run 3DMark11, the 820m's score is significantly higher than the Intel HD Graphics score (about double).

I've also turned off all throttling, and ThrottleStop shows that none of the CPU cores are being throttled. I've also set the 820m to use highest performance possible.

Anyone else seeing something similar?

Machine specs:
ASUS EeeBox EB1037
Intel Celeron J1900
Nvidia Geforce GT 820m
4 GB RAM