I am a little bit surprised by this statement. On my phone (Galaxy S20 from 2 or 3 years ago) it takes less than a second to show up. Authenticate with fingerprint. Done. I have a few hundred notes, maybe you have more.
Only caveat is the sync. Especially while shopping, every modification triggers a sync operation. So each time you (un)check an item because you just put in in your shopping basket, it syncs. It would be great to be able to temporarily disable the sync. The next best thing I found was to check "Sync only over WiFi" while shopping, so it doesn't sync. But you have to not forget to switch it back. A visible switch on the main screen would be way better. I opened a separate topic for that.
As a side note : don't forget that Joplin now supports profiles, both on Desktop and mobile. Great feature. No need to sync notes that you know you are never going to use on your mobile, make a dedicated desktop-only profile for this. And even on the mobile, you can separate your notes into several profiles, depending on your needs. So if you have a lot of notes, try and think if it would be appropriate to split them into several profiles.