Why does installing an app take so long?
DMG is a disk image file format.

Disk image files behave like a physical disk. In so doing, the operating system interprets them in the same way as if you had just inserted a CD-ROM, USB drive, etc into the computer. 

Therefore, how long an application takes to install depends almost entirely on the application itself (e.g. It's size, whether it needs an installer, etc) and other factors such as the speed of your drive.

For more info on DMG files you may want to read my answer here.