Rendered pdf takes time to load (layer by layer)

I created a brochure in Illustrator and exported it as a pdf. When I open that pdf file, it takes time to fully load, it seems to me that is opening layer by layer. It takes 2-3 sec to fully open. Also, the same problem happens when zooming. Please take a look at the attachments to better understand.

What I did in Illustrator, is that I created shadows via blend mode, and I think that's causing the problem.

So, what can I do to solve the problem? Except, to remove the shadows.

Thanks,

Goran