Hm, that sounds odd. If the thumbnails are updating, then that would mean that the drawing operations are completing as normal. The problem must be happening somewhere in the main loop is my guess. If you open the secondary viewport by pressing the V key, and make sure its' dropdown choice is set to All, if you draw in the primary viewport does it get shown in the secondary viewport?..
Yes, the secondary viewport updates normally at a speed comparable to chromium.
However, if I attempt to adjust the secondary viewport's zoom, it stops updating.
Another thing I'm wondering about is if you try dragging the resize handle of the primary viewport so that the viewport is smaller, does that affect anything?
Yes, it causes the canvas to be redrawn with the updated content, once I finish the drag.
It has no other side-effects (ie. this does not cause the canvas to begin updating normally)
What if you set the dropdown options on primary and secondary to Frame, does that affect the drawing?..
No.
Thanks for cooperating in debugging this.