What I did: I imported 1200 notes from Apple Notes
What happened: the Agenda App became very slow. It took a long time to start up. After that, menu’s appeared only after 5 seconds or so when I tried to select them (this was better after using the app for a while). I collapsed all notes, uncollapsing a note takes several seconds.
Things that might be helpful to know (Agenda version, OS and model, etc): I have an older (2012) iMac but I was expecting the app would function normally. The Apple Notes app doesn’t have a problem with 1200 notes.