Package with ~4900 lines in body, file size ~380 kb.
Interesting thing, though: if opened from the file system, switching between package header and body is fast, when opened via database (view spec and body), the switch is slow.

Can it be the showing of compiler warnings:
PLW-07206: analysis suggests that the assignment to 'xyz' may be unnecessary
PLW-07207: BULK COLLECT used without the LIMIT clause
etc.
that slows it down (14 warnings in total)?

If I compile the file into the database, it becomes slow again, so maybe it is the showing of the pane with warnings, compiler hints etc. that is slow?