I don't see any direct connection between large attachements and extended search times. The number of notes you describe can't be a factor either, because 1k notes is virtually nothing.
I think it's the increased complexity of Notes functionality combined with inefficient coding. Whereas years ago the Notes app was fast as usual despite thousands upon thousands of notes, today it is no longer so. For me, the Notes app on all my devices has long been a shadow of its former performance. So far, Apple has not resolved this issue(s). With iOS/MacOS 26, sporadic dysfunctional iCloud synchronisation added up to this heap.
Anyway, sry that I don't have a direct resolution for you.
Sidenote: How much memory do the notes take up on a Mac? See: library > Group Containers > group.com.apple.notes