Why does Time Machine take hours to back up even when used monthly?

I use a 2015 Macbook Pro (250GB SSD) and have about \~100GB in stored data. Every month, I run a Time Machine backup and literally it takes hours. On my 2017 Macbook Air with the same amount of data (\~100GB) on a 125GB SSD and it just takes a few minutes for a Time Machine backup. What am I doing wrong?