Why is my iOS Reminders app slow and unstable when invoked by Siri? I am a longtime user of the iOs "Reminders" app. And I most commonly create new reminders through Siri, for example by saying "Remind me to pay the rent at 6pm." 

These Siri requests started taking a slow, long time to actually create the reminder. 


I would make the Siri request, and then Siri would say things like "I'm on it," "Wait a moment," etc.  
A very long delay would pass. It would take up to 15 seconds before it would act.
Then, it would actually even fail about 15 percent of the time, saying "Sorry, there was a problem with the app." 
A side issue that is truly a bug but not the point of this post is that, after the Reminders app was approved in iOS13, my default reminder list appeared totally blank when viewed in the web-based iCloud reminders app. (It was OK (after an initial delay of several days) in actual mobile devices and in the macOS app.) 


What caused this slowness problem, and how to fix?
The fix to this problem is due to the fact that the reminders list that it was attempting to add to (... which is the one that was configured in iOS as the "Default reminders list") was a very old and very full list.   It was not super full of un-completed reminders, but the number of past COMPLETED reminders was over 8,000.

So, to fix the slowness, I simply created a new reminders list in my iPhone. Then I configured this new reminders list to be the "Default reminders list." 

After that point in time, Siri commands to create new reminders work smoothly and quickly.  (AND, related to that final bullet, this new reminders list was fully visible, not empty, in the iCloud web app.)