PDF's for some reason bring a huge performance hit when attached to a drawing. I do not know why other than it's a true statement. That's why I always convert to a 2-Bit TIFF file (aka monochrome TIFF).
Here is a link to an online converter that works great for small batches. A drawback that could be minor, or major, is if you utilize Vectorized PDF for the PDFOSNAP tools, you will lose this functionality obviously. Hope this helps.