Evernote for iPhone has been slow as ***** for years. I’ve used multiple iPhones with it. Everything; search, note creation, note editing - all very very slow.
If I remember correctly this started happening from about 4000 notes upwards though I’m not sure why note database size should have an affect on the performance of the app once it’s loaded. I have been forced for years to use iOS notes app for quick access and jotting of lists and ideas. Evernote iOS is a non starter for this