I've tried disabling some extensions (the ones left are things like Adblock, Collusion, LastPass, Chrome2Phone). Beyond that, I can't really think what to look for. I don't think it's just Google Maps either - perhaps it's the fact of panning around big images.
Any suggestions for ways to diagnose the problem, or possible fixes gratefully appreciated.
EDIT Further info: exactly the same symptoms in Chrome, Firefox and Opera. Using normal Google Maps is sort of ok, but apps built on it (especially Google Map Maker) are terrible. Usable for a little while, then everything in the app window gets slower and slower, including typing and moving the mouse. I'm mystified.