xthe/nthe at THE-3.2RC2 level; Regina at REXX-Regina_3.7(MT) 5.00 14 Oct 2012 level. The problem is unchanged with ooRexx.
The problem is most visible while editing large files (> 400k lines).
I have a macro that does a "down n" command and then adds a line. I set pf3 to "macro eek 3 sleepr" to insert a "sleepr (sleep random)" every 3 lines. For large files, at first this insert runs very quickly. Then it slows down a LOT. Towards the end of the file the speed increases to the initial speed.
I'm including the eek macro here. Don't ask why I named it eek... :-)