I suspect that the slowness was caused by Bash having a couple of pathological cases where it converts back and forth between the LANG/LC encoding and internal

I suspect that the slowness was caused by Bash having a couple of pathological cases where it converts back and forth between the LANG/LC encoding and internal wide-characters repeatedly, which can cause it to be stupidly slow. I suspect that setting LC_ALL=C for that portion of the code would speed Bash up to Dash speeds.