The Acrobat Reader version 9+ are slow to open even a one page PDF with 2 caracters in it. The slowness come from the side bar, (comment bar, highlight and so on).
To improve the loading speed you have the option to turn off those bar, but in some cases it’s not an option.
The way I figure out to speed it up :
3D & Multimedia
Desable : Enable Hardware Rendering for Legacy video card
Disable Enable Protected Mode at StartUp
Close all adobe instance or reboot.