It was reported on the plplot-general mailing list on 2009-10-06 by Paul Boley that plshades draws quite slowly, especially for large images. See the thread starting here: https://sourceforge.net/mailarchive/forum.php?thread_name=alpine.DEB.2.00.0910071233470.32087%40ybpnyubfg.ybpnyqbznva&forum_name=plplot-general . I thought I would try to fix it, but obviously haven't in the last 15 months, so am filing this bug report to bring it to the attention of people who might be able to speed things up.
To test it for yourself, in the x16c.c, change the default number of data points in x and y (currently 35 and 46) to something 10 or 100 times larger.
It is not uncommon for scientists to be working with 4k x 4k images these days, so waiting some large number of seconds for an image to render (especially if working in interactive mode from PDL or SciPy) makes image display with plplot very frustrating.