While upgrading Cartopy's test images (SciTools/cartopy#354), there seems to be a change in large pcolormesh clipping in 1.4.3. It is bisected down to 44ff97e (note: you also need to apply 86824c3 to fix the typo), which does achieve the desired speedup, but also changes clipping slightly:

It does not just affect the above because of the strange shape. Even the simpler Mercator projection ends up clipped differently: