I’ve found a working solution: if I zoom into a map section of around 12x6km and then start the edit Potlatch works at normal speed. If I start editing with a map of around 30x15km the rotating wheel on the top left goes on for ages (it’s still at it after 5 minutes) and no map components appear. So it seems that the data quantity needed for the 30x15 map overwhelms Potlatch. But that’s no problem. Now that I know the cause I just start with a small map segment.

Curiously, when I start wth a small map segment and then zoom out Potlatch no longer has a problem loading the data.

It would be nice to have a little “Stop loading this page” button in Potlatch similar to the one in the browser so that when it gets into this kind of problem one can simply tell it to stop trying (rather than kill off the browser page, which is my current solution).