Performance bug introduced by 3.0 and the new rendering engine: the rendering engine lazily computes smoothed VBOs on demand when a part is drawn, and the smooth is done on a full-library-part basis.
The result is that when we change MPD submodels and a large number of parts are drawn for the first time, there can be significant delay (Seconds) before the screen updates.
I would say this is not a stop-ship for 3.0, particularly since fixing it requires significant re-architecture of the part library, which is well beyond our remaining risk profile for 3.0.
The fix is to allow cached smoothing of sub-parts; doing so will significantly reduce smoothing computational cost.
(A hack would also be to force-draw every single part in the model once on load, to 'warm' the entire model.)