I tried to repro a worst case based on your info:
as you can see, 20 pages full of errors, spell and grammar correction enabled, writing in the first page. It clearly shows more CPU usage than the first test, but always below 40% for every core.
Please, note that I am not trying to say that you did not hit a bug or even a faulty device, just that I have not been able to repro it in my limited testing and so far the device is totally impressing me. I am even thinking to write a cool Hackers News app for it :-).
I am a really happy camper, today I tested the RDP connection to my main server and works really well, you can get real stuff done using remote desktop. I was sure I would need the Surface Pro and now I am not so sure as I can do heavy lifting with more battery life and lighter device (and I will always need a "real" computer in the home anyway). Surface Pro looks awesome for pen and I am a big fan of ink well done (need to test a capacitive pen with the RT but usually they do not meet my bar). In any case, I will surely keep my unit, I did not feel so happy with a new gadget since the ZX spectrum, my first HP48 (yes, I am that old) or the iphone. I was not sure after reading some of the early reviews but this device is a geek's playground, I will be discovering new tricks for a while, the connection to the XBOX is pretty cool too and seems to work fine for movie navigation but did not play much with it yet.