...it's still not actually being done by SSIS... it's being done by a script.
Well, I think we are drifting into a grey semantic area here.
SSIS natively provides the flexibility to use scripts as part of packages - they are fully integrated and not some sort of add-on or hack. In this case, the data pipeline runs through a script which processes each row in (a pre-sorted) order and uses variables to track row numbers. It's the pre-sort and row-by-row processing that slow things down, of course.
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.