Like you, I used to use Age of Empires III: Asian Dynasty as a test article for a new built machine because the game would rip the guts out of the video system.
There is a possibility that a throttling action is being taken via the BIOS. That could be in place to safeguard the tablet. The "Haswell" series of processors (to include Pentium and Celeron versions as well) are real good and can clock in real fast in a desktop. I have both the i5 and i7 versions in desktops now and have three motherboards to build up the Pentium version.
With a desktop and laptop, you don't need to control the speed of the processor (which I think is happening here) because they both have robust motherboards and chipsets to support that. The tablet versions will have the high speed performance but with limits. That means, no horsepower to back it up. The supporting chipsets are probably key to the issue here and a limit has to be set.
Now, when PC makers mention "gaming", that can be subject to interpretation. Playing Solitaire is a game! I think you know where I am going on this one.
Don't get rid of the tablet. There can be tweaks to increase performance in the future. The Pro 3 is a re-engineered machine compared to the 1st and 2nd generation models. I would expect more firmware updates in the future to do that.