I’ve noticed strange navigation behaviour if you open map with terrain: camera rotates around itself (not around a point on the ground), pans and zooms slowly.
It looks like that picked point is very close to display, not ground. You can see this in https://d3h9zulrmcj1j6.cloudfront.net/Marseille_Cesium/App/index.html
or in sandcastle on examples with terrain. If you wait until terrain fully loaded this bug will disappear
Especially it’s noticable if you use Firefox (for example in v.53.0.3 )