I've noticed that when I start sketching walls in a new project that at the third segment Revit decides it needs to PAUSE before continuing. It seems it has something to do with having Revit Extensions installed. It thinks needs to add some parameters and interrupts my sketching to do that. Bill (Mr. BIM Thoughts) helped point me toward this bugger the other day when were discussing a few workstations that were pausing to install/update the extension...much worse pause.
If you experience this too...might be the same situation for you.