Why does backing up an iPhone take so long?

My 1Phone 12 Mini has been warning me for days that its last backup was more than three weeks ago. It kindly explains that backups occur when the phone is locked, connected to power and on a WiFi network. Mine is locked, connected to power and on a WiFi network every night. But when I expressly set it to back up again in response to these warnings, it begans by telling me that the estimated time required five hours, which then expanded to 11 hours and now, three hours later, says it still has 14 hours to go. There’s less than 40GB of data, about a sixth of the phone’s capacity. What gives? I see that this question has been asked by numerous people over several years and has been allowed to go unanswered.