A confirmed bug with the Pocket Lists 2.8.6 version for iOS (as of April 2020) — on iOS 13.3.x, a significant few-seconds delay is observed when tapping on any to-do list in the iOS app.
How to fix: simply update the iOS version on your device to 13.4+. The lag will go away automatically.
The update with the fix is on its way and will be available on the App Store shortly!