Imaging has a lot a variables that affect its speed. The compression ratio selected; the higher the compression, the slower the backup.
Imaging to a second local drive is always faster and safer than creating an image on the source drive. If you are storing the image on an external USB 2.0 drive, the backup time will be signifigantly slower due the speed of USB 2.0 transfer rate. Imaging to my external USB 3.0 drive on the other hand is almost as fast as to my internal SATA 6 GB/s backup drive.
I guess I should have mentioned that my image backup times are based on a fast SATA 3 GB/s source drive to a separate destination internal SATA 6GB/s drive.
Finally there is the imaging software used. As the saying goes, not all are created equal.