A couple of people on twitter share this issue, but I cannot find any discussions of it nor solutions.
Since moving to OS X 10.9, Reminders app keeps pinwheeling, and won't respond. It doesn't crash, so I have to force-quit it. This happens on both my 2008 iMac and my 2013 Air. On my Air, I can tell something weird is happening because upon launching Reminders the fans kick on loud.
The data seems to be syncing fine from iCloud, so it doesn't look like a sync issue is causing this.
Any ideas about what could be causing this issue and how to fix it?