Maybe it goes faster when you use a JPEG file (if that is an option).
JPEG files are copied into the PDF as they are. PNGs and other formats have to be converted into "PDF images".
You can use pages from PDF files just like images. This is another option you can try: once create a PDF with your background image, and create all other files with that PDF instead of the PNG (if JPEG is not appropriate for your image).
There are two builds of MigraDoc: one using GDI+, one using WPF. You could try both to see if that makes a difference.
BTW: Images can be positioned like TextFrames, so there is no need to put an Image into a TextFrame.