Default is 150 DPI, which is crisp on screen and fine for most previews. The advanced options expose 72 DPI (small web thumbnails), 150 (default), and 300 DPI (print-sharp). Higher DPI gives a sharper BMP but a larger file and slower render.
Page rendering is fast — roughly 0.2-0.5 seconds per page at 150 DPI, so a 50-page EPUB finishes in well under a minute. Higher DPI multiplies the time. Premium runs more parallel render workers for large documents.