These two changes seem to speed up rasterizing in the sequence grid.  The IntentRasterizer.cs change just uses a solid rectangle when possible.  The EffectRasterizer.cs change is a bigger departure.. basically, it limites the number of elements being rasterized to 15 or so (rather than rasterizing a Y row for all elements).  I think is still looks pretty good and runs a whole lot faster.  Try it on Giz's sequence with AllElements open... note how long it takes to Ctrl-+ before and after this change.