The superseded MacBook Pro’s is backed up to a faulty ext HDD. The fault is in the interface between the micro USB 3 port and the HDD circuitry itself. There’s no traditional SATA connection as such, from HDD’s bottom surface, there are a few terminals linking back to the USB port. The fault is such that the extra connections in a micro USB 3 port vs those of a micro USB 2 port are not making connection & data transfer is at USB 2 speeds. This drive was last used in Sept 2020 when it was replaced by better & faster drives. I use those newer drives are now being used for Time Machine & clone backups of the M1 MacBook Air.
It’s a 2 TB drive partitioned into 1.5 TB for Time Machine & 500 GB for a clone. The MacBook Pro has a basic version of Monterey installed, occupying 37 GB in total, whereas with the M1 MacBook Air the Music folder alone is almost that size. When the drive was backing up the Mac in 2020, it was running Catalina and Time Machine was not encrypted.
But for Time Machine to work properly under Monterey, the drive has to be encrypted &/ that's what's taking so long. At the current rate of "progress" it might be completed in four more days, even though it's at least 70% of the way through the encryption process. This is because the damaged micro USB 3 interface can only work at a maximum of USB 2 speed, up to 480 Mb/s.