In general I don’t think custom render loops are necessary, and they won’t really provide any performance improvement. Cesium’s preRender and postRender hooks generally give you all you need to do any custom render-time processing. I think that Doarama uses a custom render loop primarily because that code was written before Cesium even had preRender and postRender. Hopefully Chris Cooper will chime in if that’s wrong since I haven’t worked on Doarama myself (I’m only a fan, and I sit next to Chris at work, but he’s not here at the moment or I’d ask him!)…
The other thing about Doarama, though, is it doesn’t use the entity system. Partially that’s because it was written before there was an entity system (seeing a pattern?) but partially it’s because Doarama does some tricky things and wants a level of control that is not possible with entities. For example, I believe the GPS tracks are rendered as completely static polylines, and a custom shader with time passed in as a uniform makes the line appear incrementally over time.