I have two calendars as .ics files that I synchronize (externally via vdirsyncer) and that are both set as external files in orage. The bug persists in both Xubuntu 16.04 and 18.04.
When opening orage, it takes several tens of seconds. Whenever performing an action in orage (e.g., opening a menu), it also takes several seconds, with a faster hard drive making it a bit faster though. When I put an strace to orage, then I see that for every action, it will load one of the calendar files byte-by-byte, until it performs the action.
When I remote the second calendar, everything reacts instantly.

This effectively makes orage unuseful for me for the use case of having two calendars.