Update: you were right, and I finally reproduced it. Late in a run the town map was dropping to 6 to 10 frames a second in the browser. Every house, road, lamp and window glow was drawn separately, and the browser charges a little for each one, so the more of the town you bought, the slower it got. That is why it crept up the longer you played, and why my desktop tests never saw it.
v0.1.10 is up now. The whole map is drawn once into a single image and only redrawn when the town changes, so it went from about 490 draws a frame to 11. In my browser test it now holds a steady 60 while dragging. The same fix is in the full game too.
Thanks for sticking with it and for not letting it go. If you still see a stutter anywhere, the backquote readout screenshot is the fastest way for me to find it.

