It depends on the size and length of the file. Small documents and images are usually ready quickly. Long videos take the longest, because most video conversions re-encode every frame.
You are viewing a raster image, so zooming past its native resolution shows the pixels. An EPUB renders at a fixed 150 DPI here. For a sharper BMP, convert the EPUB to PDF and use the PDF to BMP converter at 300 DPI.