Worked it out.
In the admin panel:
then tick the 'upload files' box for registered users.
our infrastructure currently can not support websockets.
But polling websockets consumes 100% CPU, as it seems.
What is the simplest was to disable websockets completely, so that the browser stops polling?
Can you let me know more about your infrastructure, and how it cannot support websockets?
Disabling socket.io altogether will break certain functionality...
Also, long polling should not be consuming your CPU 100%. It sounds like something else is going on.