Good idea, hadn't considered that as a potential issue
Looking at out database events table we have about 600 events in total, including assignment deadlines etc. I didn't think thats excessive but maybe there's something in the site settings which is exacerbating the issue.
Because of this events issue, I've been holding off creating events for the 15/16 academic year so there's nothing but one test event for September onwards, but looking at June 2015 we had 30 events in total for the month.
May or may not be relevant that we have the calendar lookahead set to 1 year, versus default of 3 weeks (see screenshot below). I thought this only affected the "latest events" block. I have just tried reducing this value to 2 months and creating a new event within a course but I still couldn't see it as admin. We also have calendar export lookahead and lookback set to 365 days, but again I understood that was only for iCal exports.
I suppose the next logical step might be to start deleting old events and seeing if new , but I'm loath to do this as they're a part of our archiving records. Does 600 events sound high enough to cause issues? If its relevant we're using the Essential theme.