Hah, profiling code reveals Surprises, again. In Blender VSE timeline area, there are strips with thumbnails and waveforms and curve overlays, text labels, rounded corners, borders, editing handles and whatnot. Yet, what takes up 20% of the time to render the timeline? That's right, drawing this list of "channels" on the left side. Because Complexicated Reasons that end up doing not-too-scalable things as timeline complexity grows :/