I've got SuperDuper running on a Mac Mini which is running 10.11.6.
It has, for a long time, run a nightly backup. It took about 30 minutes. Here's some excerpts from the logs:
2018-07-25 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-07-25 03.30.26 -0400.txt:| 03:52:29 AM | Info | Copy complete.
2018-07-26 03.30.27 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-07-26 03.30.27 -0400.txt:| 03:56:00 AM | Info | Copy complete.
2018-07-27 03.30.21 -0400.txt:| 03:30:22 AM | Info | PHASE: 1. Prepare to Copy Files
2018-07-27 03.30.21 -0400.txt:| 03:51:59 AM | Info | Copy complete.
2018-07-28 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-07-28 03.30.26 -0400.txt:| 03:51:14 AM | Info | Copy complete.
But recently it's taking a lot longer:
2018-09-10 03.30.21 -0400.txt:| 03:30:22 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-10 03.30.21 -0400.txt:| 05:55:03 AM | Info | Copy complete.
2018-09-11 03.30.27 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-11 03.30.27 -0400.txt:| 07:22:07 AM | Info | Copy complete.
2018-09-12 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-12 03.30.26 -0400.txt:| 06:35:45 AM | Info | Copy complete.
2018-09-13 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-13 03.30.26 -0400.txt:| 06:03:31 AM | Info | Copy complete.
2018-09-14 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-14 03.30.26 -0400.txt:| 06:28:44 AM | Info | Copy complete.
2018-09-15 03.30.27 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-15 03.30.27 -0400.txt:| 07:52:19 AM | Info | Copy complete.
2018-09-16 17.24.04 -0400.txt:| 05:24:05 PM | Info | PHASE: 1. Prepare to Copy Files
2018-09-16 17.24.04 -0400.txt:| 09:40:42 PM | Info | Copy complete.
2018-09-17 03.30.27 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-17 03.30.27 -0400.txt:| 05:31:10 AM | Info | Copy complete.
2018-09-18 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-18 03.30.26 -0400.txt:| 06:17:47 AM | Info | Copy complete.
2018-09-19 03.30.23 -0400.txt:| 03:30:24 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-19 03.30.23 -0400.txt:| 06:46:08 AM | Info | Copy complete.
2018-09-20 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-20 03.30.26 -0400.txt:| 08:01:27 AM | Info | Copy complete.
2018-09-21 03.30.26 -0400.txt:| 03:30:27 AM | Info | PHASE: 1. Prepare to Copy Files
2018-09-21 03.30.26 -0400.txt:| 10:45:04 AM | Info | Copy complete.
I don't see anything in the logs that gives me an idea of why it's taking so much longer, and I've checked the drives with Disk Warrior to make sure there aren't any problems with the drives, but it definitely seems like something must be wrong, doesn't it?
I've put a copy of the logs here:
https://www.dropbox.com/sh/uya6jr3xb...d3d0AweCa?dl=0
(There's a zip and a .tar.xz. The contents are identical, but the .tar.xz is a lot smaller if you have The Unarchiver or 'xz' installed to open it.)