What is Time Machine doing?
I love Time Machine. It has saved me almost a dozen times over the last six years. For the last year or so, however, the hourly backups have grown larger, and take longer, for no apparent reason. I have excluded useless things that change frequently like the Safari Cache. The documents with which I work are usually not large, ranging from a few K to 10G if there is a lot of graphics. Yet TM finds 200 to 400 GB to copy every hour! Why?
My backup drive is connected by a Thunderbolt cable.
iMac, macOS High Sierra (10.13.3), mid-2011, 16GB RAM