Project Tango has a 320x180 depth sensor running at 5 fps, i.e. 290k depth measurements per second. Compare this with off-the-shelf depth cameras (e.g. DS325) that generate 320x240 at 60 fps, i.e. 4.6m measurements per second.

The reason for this is that mobile processors aren't fast enough to process more information. So the Tango prototype has to have, in addition to its depth camera, a special motion tracking camera with a fish-eye lens and 2 dedicated processors in order to robustly track.

Even then, with less depth information, the quality of any Tango reconstruction will be far inferior. Maybe in 5-10 years, mobile processors can approach what desktop GPU's are capable of today.

In any case, it remains to be seen if Google can persuade cellphone manufacturers to include 2 special cameras + 2 extra processors in their future devices.