remindd sometimes spikes over 100-200% CPU usage
What worked for me is to turn off Reminders syncing, reboot, then turn it back on. Assuming you are running Catalina, you would go to:
System Preferences -&gt; Internet Accounts -&gt; iCloud -&gt; Uncheck Reminders
It took a bit for my reminders to sync again, but after that, there were no more issues with Reminders app lockup and the remindd run away CPU usage.
However, as of recent (Catalina 10.15.7), remindd has run away CPU usage regardless if I turn off/on syncing or I just leave it off the all the time.
Since I posted this answer, I will add that even with Big Sur (including 11.3 update), so long as I have Reminders enabled, I get high CPU usage with remindd. In addition, reminders are not properly syncing between my iPhone, iPad, and Mac. It appears there is some corruption in my Reminder data and it is not getting fixed with macOS updates.