I think it'd be curious to see the code, some example strings, and whether or not you compiled with optimizations enabled — if you're willing to share. There's no reason for this to have been the case.
As a dataset, I just use find to generate a list of paths from the file system:
find / > large.txt
I tested with Swift 5.7 and 5.9, and Python is always between 4x and 10x faster.