Zoom is slower when close to a model

Reported in the forum here: http://cesiumjs.org/forum.html#!msg/cesium-dev/C_OT9kB9RV8/-zlIMRjWCAAJ

Zooming is slower when the camera is close-ish to a model. Possibly not the proximity to the model, but the fact that the model is "in the center" of the view.

http://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Hello%20World.html&label=Showcases&gist=a4bc90cb451cc3c997566e34a655c5ca

if you change the value of `latOffset`in `getPositionCentered()` you will notice the difference.

Start with the linked Cesium Sandcastle Hello World example and reproduce the slowdown while zooming near the model. Compare behavior after changing latOffset in getPositionCentered(), then trace the camera zoom path to identify the performance difference. Done means zooming remains responsive in the reported near-model and centered-model cases.

Written by the indexing model from the issue text.