You’d probably have to take a look yourself, for example in Task Manager → Performance, especially your CPU and Disk utilization. If you have a mechanical hard drive (not an SSD), it’s very easy to keep it busy by checking lots of disk files – which pretty much by definition is what a backup has to do to find changed files, then back up. There are ways to lighten the impact, for those who want to slow Duplicati to favor other work.
HDD-access speed during backup speaks mostly to reducing the impact ot disk accesses, but I think it gets into CPU a bit. It also doesn’t get into the –usn-policy option which reduces disk use of finding changed files. FailedToUseChangeJournal covers Administrator permissions it may need for a non-service Duplicati install.
With PC resources being shared, you might also notice different sorts of “running slow” depending on what you’re running. Disk-very-busy might not bother a CPU-intensive program much, but would slow a disk-user.