Does it read the entire script into memory FIRST even if it's over 1MB

No idea but to me that's not the over-riding factor. If your comment section is 1MB and the mean line length is 60 bytes then that's coming in at over 17k lines of comment block. I'd say that's too large. Pity the poor monk who has to do a search+replace in all of that only to find that most of it is comments. Move it out into a separate file and you'll keep everyone happy: both the POD editors and the code editors.