Using rsvg as SVG renderer in the tests is much faster than using inkscape: with the following simple patch:

diff --git a/lib/matplotlib/testing/compare.py b/lib/matplotlib/testing/compare.py
index ab2f26fc0..a90a75901 100644
--- a/lib/matplotlib/testing/compare.py
+++ b/lib/matplotlib/testing/compare.py
@@ -138,9 +138,16 @@ def _update_converter():
         converter['eps'] = make_external_conversion_command(cmd)
 
     if matplotlib.checkdep_inkscape() is not None:
-        def cmd(old, new):
-            return [str('inkscape'), '-z', old, '--export-png', new]
-        converter['svg'] = make_external_conversion_command(cmd)
+        import cairo
+        from gi.repository import Rsvg
+        def convert(old, new):
+            svg = Rsvg.Handle().new_from_file(old)
+            svg.set_dpi(96)
+            surf = cairo.ImageSurface(
+                cairo.FORMAT_ARGB32, svg.props.width, svg.props.height)
+            svg.render_cairo(cairo.Context(surf))
+            surf.write_to_png(new)
+        converter['svg'] = convert
 
 
 #: A dictionary that maps filename extensions to functions which

the entire test suite runs in 594s instead of 787s on my laptop. (There's also a ton of image comparison failures, but that's because my cache (which I did not regenerate in either case) is generated by inkscape and there seems to be pixel-level differences between the two renderers -- but that won't be a problem in practice as all we'll want to do is to compare svgs rendered by the same engine.)

There are also a few constructs that are not correctly rendered by rsvg but perhaps it's a good opportunity to try to generate slightly less demanding svgs... and again in any case we're still comparing the same misrendering by the same engine :-)

The real problem right now is whether there is an easy way to install rsvg on Windows. There is an official .exe installer on Windows (https://wiki.gnome.org/action/show/Projects/PyGObject?action=show&redirect=PyGObject) which I haven't tried; I haven't found a pip package; there are a few user-submitted conda packages (search for pygobject/pygobject3) but none of them seem to work on Py3.

Alternatively, I have tried two other renderers: QtSVG (http://doc.qt.io/qt-5/qtsvg-index.html) and cairosvg (http://cairosvg.org/, which only supports Py3, but could be used as a standalone script that lives in its own environment, similarly to inkscape). QtSVG is a very, very limited SVG renderer so I'd rather not use it (unless we can make our svg output compatible with it); cairosvg seems to freeze(?) on at least one of our constructs.