I'll try your suggestion to set whatever backup software priority to high.I have used the Windows image backup for a long time. On Win10 Pro it has slowed down. Switching to a 3rd party app has issues if your PC uses secure boot. A number of 3rd party apps are not able to create startup CDs or USB drives that work with secure boot. I found the solution to the issue was managing priority of the image backup run. Using task manager to set priority to high solved the problem. Performance impact using other apps while running the image backup in high was noticeable but not worth concerns. Usually I run the image backup in the evening after finishing work. It now runs in 3.5 hours in a 3 partition setup of roughly 500gb. I will stay with Win10 for the image as the Win10 startup disk does work in secure boot mode.
Right now I'm using Macrium to create a clone hard drive installed in an external USB-3 docking station with some relatively fast sucess. For example, yesterday evening it took only 1 hr and 14 minutes to create the clone. Backup and Restore (Windows 7) was never able to match that. The next time I do this I'll set Macrium priority to high and see if that speeds things up.
BTW, I'm creating a clone rather than an image in order to produce a hard disk that I can use to just replace the PC's hard disk when, not if, it inevitably fails. (I presume with a clone that no recovery disk or thumb drive would be necessary; just install the new clone of the original drive and hit the power button.)
To just exactly WHAT are you backing up -- an external hard disk, an internal disk, what?