wx.Process slow down

I'm having a problem with a long running process, basically
using the code from the demo under Process & Events/ Process.

Using the same code to output a long running process to
a TextCtrl works fine for a few lines of text at a time, but
if the external process ouputs a lot of text quickly the app
gets slower and slower until it freezes for a few minutes then
finishes (the external command only takes a couple seconds
to output in an xterm).

To demonstrate the problem, you can go to the demo and remove
this from the input line that says "Command line:"
python -u data/echo.py

and type in
ls
(or something in Windows that outputs a few lines of text)

That works great. But if you do a command like
ls -laR /tmp

or something that outputs a lot of text, you'll see it
starts out fine but gets slower and slower.

Any ideas how to get around this? I get the same behaviour
with wxGTK & wxPython 2.4.2.4 and 2.5.3.1

Thanks