Gallery: spectral graph partition very slow

[rossbar]

Just opening this issue to track a performance issue I noticed with the docs.

The image segmentation and spectral graph example added in #7040 is extremely slow - it takes about 100s to run on a fast machine (ryzen 5950X) and is even worse in CI, taking nearly 4 minutes per doc build to run the example.

This should be investigated ASAP to reduce CI burden. There are multiple 3D animations in the example - I suspect switching them to static images would resolve the issue, though there will have to be a bit of profiling to verify that the animation(s) are the bottleneck and not the analysis itself.