Time Machine can’t get past “Preparing backup”

I’ve tried 4 times now to do a Time Machine backup. The first 3 times, I waited a half hour for it to get past “Preparing backup” before stopping it. The last time, I unplugged the external hard drive and plugged it back in, and tried again. Half hour, still preparing.

Notes:

macOS Catalina 10.15.4

The external drive is 2 TB, capacity, with 1.5 TB free.

The iMac has a 1 TB fusion drive, with 133 GB used.

The last Time Machine backup was on March 20, with about 3 hours usage since then, no new applications installed. Maybe a couple updates, new data.

I have to say I’m surprised. I expect this kind of “hanging” behavior from Windoze, not macOS. Am I expecting too much? It has taken a long time to prepare the backup the last few times I used Time Machine, but never this long. It used to take just a few minutes to prepare, maybe 5 or so.

Anybody have any ideas or comments?

ADDENDUM: It finally started backing up after about 40 minutes of preparing. I stopped the previous attempts after about 30 minutes. Is this what I should expect from now on? Is 40 minutes the new 5 minutes?