Well, if you are reasonably sure you won't need to edit your brush strokes anymore, you can use the flattener to create a rasterized image. Select the objects and do a Object(s) --> Rasterize. AI isn't the greatest at dealing with raster data, either, but at least that should eliminate the constant re-calculating of the blend operations and rasterizing complex paths for on-screen display. Just make sure that you save an editable copy of your file with all your brushes as vectors, just in case you need to go back and change something.
Mylenium