Try turning down the Document Raster Effects Setting from Effects>Document Raster Effects to 72 from what ever it is now. The Blob Brush is a raster effect that has to be rendered. If the setting is too high is will slow things down. You can change it back to something higher for export. You might also want to explore getting more RAM. 4GB is a bit low for working with the new versions of the OS AND AI.
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