I'm on the Word team in Office and I'm coming into this thread incredibly late, but FWIW the slow typing on ARM was something we couldn't fully address in time for the preview release. Word RTM (ARM and x86/x64) has much better performance.
One of the challenges we had was that for a large majority of the product cycle we didn't even know about the Surface. We were looking at ARM early on, but the hardware we had was prerelease hardware from MSFT partners that had varying levels of performance.
Even now we don't know a whole lot about the Surface. It's a little frustrating, since we (Office) hit RTM we're eagerly awaiting the point at which new Surface RTs will have Office RTM baked into them. The preview release is seriously many many months old from the RTM release, and it's painful to think that this is what customers are going to see from Office when they turn on their shiny new device.