Emacs 29 slow subprocess output when using pipe for connection

[Chris Hanson <cph@chris-hanson.org>]

On Emacs 29, I've noticed when using the MIT/GNU Scheme interaction mode 
(xscheme.el) the output is extremely slow; it was fine in Emacs 28.

After a few hours of debugging, I found a fix.  In 
xscheme-start-process, I changed the following lines:

(setq process
       (let ((process-connection-type nil))
	(apply 'start-process
	       (cons the-process
		     (cons buffer
			   (xscheme-parse-command-line
			    command-line))))))

to

(setq process
       (apply 'start-process
	     (cons the-process
		   (cons buffer
			 (xscheme-parse-command-line
			  command-line)))))

This simply changes the connection type from pipe to pty.

That's surprising behavior.  Why is the pipe connection so slow?  Has no 
one noticed this because the default is to use ptys?

Any input would be helpful.  If there's no feedback, I'll file a bug.

Note that I don't read this list, so please include me in any reply.

Thanks,
Chris