To be clear -- I'm not saying you shouldn't use LLMs for this sort of thing. Their potential utility is obvious and, to be honest, I make just as many mistakes and often write far ropier code than they output!
What I am urging you to do is at least "sniff test" the code before you run it. AppleScript is very "readable", it mainly does what it says, and it would be sensible to at least look for an accidental "then delete all my files..." before you hit the "Execute" button...
I haven't done it myself, but I've heard good reports about the "...and now explain this code..." method, where you ask the LLM to explain its output before you use it. That, some common sense, and a little practice should protect you from the most egregious errors. Optimisation etc is just fluff for most of what we're doing -- good practice, of course, but a working solution now is usually far better than an optimal solution next week 
With that disclaimer out of the way...
Doing things in/to Mail will take as long as it takes. Similarly for the two search-and-replace routines -- and "text item delimiter" operations are blazingly fast on shorter strings like these, so probably not worth redoing as calls to the KM Engine regex operation.
So it's really those multiple do shell script calls. And while, for example, doing a Finder-based test:
tell application "Finder"
if not (exists (POSIX file folderDT as text)) then
do shell script "mkdir -p " & quoted form of folderDT
end if
end tell
...takes a third of the time that the simpler
-- Full path to the target folder
set FolderDT to "/Users/bernshanfield2/Desktop/" -- Absolute full path
-- Ensure the folder exists
do shell script "mkdir -p " & quoted form of FolderDT
...takes whenever the folder is already present, we're still only talking fractions of a second for a single op.
So you could look at replacing the shell script functions with Finder-based equivalents wherever possible but I don't know if, even all added together, they'd account for your "1-2 second delay".
Have you tried just putting the AppleScript into the action, rather than reading it from a file? It's an easy thing to check, and may give you the biggest bang for your buck.