If your system doesn't buffer streams that are fdopen'd,
apply the following patch.  It makes a tremendous difference.

*** deliver.c.orig	Wed Jul 23 18:27:04 1986
--- deliver.c	Tue Nov 24 14:39:10 1987
***************
*** 694,699 ****
--- 694,700 ----
  	FILE *mfile;
  	FILE *rfile;
  	extern FILE *fdopen();
+ 	static char mbuffer[BUFSIZ];
  
  # ifdef DEBUG
  	if (tTd(11, 1))
***************
*** 910,915 ****
--- 925,931 ----
  
  	(void) close(mpvect[0]);
  	mfile = fdopen(mpvect[1], "w");
+ 	setbuffer(mfile, mbuffer, BUFSIZ);
  	if (clever)
  	{
  		(void) close(rpvect[1]);