I think it's safe to say it's slower, but as I used to remind the engineers when they pushed back against releasing slow solutions when I was working as a liason between customers and engineering: "not working is infinitely slow".
It's easy to see how Microsoft wants this for the Surface Phone since it will allow customers to run x86 applications on their phone. If your choice is between not running an application and running it slowly, most people will always choose "slowly". Most Win32 software isn't high performance software. And a lot of the really computationally hard stuff is getting moved to GPU anyway. So for instance photoshop has shifted much of its filtering and canvas rendering to OpenCL/CUDA. The Snapdragon should have a perfectly adequate if not superior OpenCL benchmark compared to a low-end intel built in GPU. Same with games. Even if you take a pretty large performance hit on CPU cycles, usually it's the GPU bottlenecking not the CPU. So their magic emulator should natively execute all DirectX/OpenGL calls on the GPU anyway.
The Xbox team has done a great job of this on the Xbox One backwards compatibility for PowerPC RISC support on the Xbox One.