That's a good point: does the bandwidth reach the native performance of another computer? Can a commodity phone/tablet stream much better performance?
However ergonomics is a different matter... Touch screen is pretty bad for desktop usage.
Android devices let you connect a USB or a Bluetooth keyboard and mouse. You get even a mouse pointer last I tried. Although not using it like that in everyday life. It's more convenient to just use a small laptop.
With iOS Apple's Bluetooth keyboard works. Other input devices didn't work, or at least I couldn't figure out how.