You can merge to a new document instead of merging to the printer, then
print the document using Word in the usual way. (I suppose you could print
directly to a .prn using the "Print to file" box in the print dialog box but
I doubt it will solve this performance problem - quite a few people report
that merge slows down as the record count reaches a few hundred records or
so).

If outputting to a new document also takes a long time - and my guess is
that it will - I would consider
a. doing an experiment where you create a new mail merge main document
(letter) with the fields you need and some test plain text, and see if that
also merges slowly. If not, it is probably worth gradually reconstructing
your mail merge main document to see if anything in particular is causing
the problem (for example, "formatted bullets" used to affect merge
performance quite dramatically - I do not know if they still do).
b. splitting your merge into smaller chunks (say 500 records per merge).
This is typically a bit simpler if you are merging manually rather than
trying to set up an automated process, but the thing to do it experiment
first, decide what to do next.

Peter Jamieson