Why is a smaller folder taking longer to be backed up than a largerone?

Okay, so I do daily Macrium file backups of my entire User folder, with
monthly fulls at the beginning of each month. All except one specific
subfolder which takes longer than all of the rest of the User folder
combined to backup. So I've excluded it, and I back it up in a separate
backup job which only runs twice a month, because I can't afford the
time to run that daily.

The User folder stats (minus excluded folder) are this:

Total Number of Files: 342862
Total Size: 913.78 GB
Backup Completed Successfully in 05:02:40

Now the excluded folder stats are this:
Total Number of Files: 651658
Total Size: 1.57 GB
Backup Completed Successfully in 11:47:09

As you can see, an approximately 1 TB folder is fully backed up in about
a mere 5 hours, whereas a puny 1.5 GB folder (almost 600x smaller!) can
only be fully backed up in about 12 hours? The only difference is that
there are about twice as many files in the smaller folder than the
larger folder. Both folders are on the exact same physical drive (HDD,
not SSD). File system is NTFS. What could be causing such a drastic
slowdown?

Yousuf Khan