Windows on ARM by its very nature will use emulation to accomplish much of what it does to run traditional x86 programs. Even with APIs and the OS kernel being ported to ARM, there will inevitably be cases where running x86 software will be slow and inefficient when translating some code sequence compared to running it on native x86 hardware.
Besides, we already know from the recent benchmarks that the latest ARM-based Qualcomm Snapdragon 830 processor, which will be the first to run Windows on ARM, is slower than Core M when each is running its native software. Take away the native software from ARM and add an emulation layer of top of this, even the very best form of emulation the world has ever known, and there inevitably will still be slowdowns.
There is no magic bullet solution to emulation. With the single emulation demo we saw of Photoshop running on Windows on ARM, they just ran a very simple filter plugin on top of an image. We still have no idea how the performance will be with more demanding, time critical tasks like real-time digital pen input via this Windows on ARM emulation but it will surely be no where close to as fast as Intel Core M running x86 applications natively.
Note that in this live demo the Windows 10 GUI and the system apps (like Microsoft Edge and the Video app) have all likely been directly compiled for ARM. The real test would have been seeing more plug-in demos in Photoshop (an x86 program) and possibly real-time digital pen input in Photoshop, which they never actually show in the video. The drop-down menus (common to most Windows applications) run fast in Photoshop since that menu system relies on a Windows API which has likely been ported to ARM to achieve such smoothness.
Seeing how more complex plug-in effects than a simple radial blur and drawing on-screen with digital pen input work, which would inevitably have some elements that rely on x86 code, would have been the true test for performance. It is very likely this demo of Windows on ARM was cherry-picking examples while it did not us many of the inherent problems which always occur when emulating any platform. Also note that Universal Windows Platform apps have easily been able to be cross-compiled to ARM ever since Windows 8 hit the market, so World of Tanks: Blitz was likely running natively as an actual ARM-compiled app, not via emulation.