Visual Editor Always Loading

I have a question about the visual editor. I can use it to change one or two things, and changes are reflected in the page immediately. However when I keep changing things, the visual editor mostly keeps loading forever, so I have to close the page and reopen the visual editor to see my last changes.

In one case (when I tried to change the logo used in e-mails) it is not even possible to save that change. The editor keeps loading forever and when I reopen the page that option has not changed.

Any ideas?