In both my work and personal Macbooks, the Messages app often bounces for a few mins before opening. Sometimes it’ll open but the window will be completely blank. Eventually though it opens but it takes a few tries for quitting and reopening.
This is in both a 2015 Macbook Pro and a 2019 Macbook Pro (16 inch). All other apps open fine.
Is this because I may have too many messages?
It would help to say what specific Macs you have, what versions of macOS are running, how much RAM you have installed, if you have many other apps running, if you’ve run any file maintenance utilities (eg Disk Utility), etc. And yes, how many notes you have would be useful to know too!
So any responses would be moderately blind guesses.
If it’s multiple Macs and it’s not the number of notes it plausibly involves connectivity to iCloud. (Even if not it could be corrected with the help of iCloud.)
I’d run Disk Utility first. Can’t hurt.
You could turn off iCloud Notes sync under System Preferences > iCloud, and turn the Notes toggle switch off and see if launching improves.
If it does you can then go from there.
If the problem persists, as a possible fix I’d consider turning off iCloud on both machines, deleting all notes from both Macs, then turning iCloud back on to let the notes repopulate to the Macs from the cloud. (You could first log into iCloud.com and confirm that all the notes are there if you like.)
But a little more info might be helpful before trying that.