Until we will do some exact measurement here is how you can have an look at your own System and see that the defragmet tool uses less resources.
You open Windows Task Manager and Resource Monitor, then you start Disk Defragmenter.
You will see at the processes in Task Manager the process DfrgNtfs.exe You can right click on it, go to Set Priority and you will see that in the time of the Deframent the Priority is set by default to Low.
The second thing that you can view is in Resource Monitor at the Disk window. Here you can see that the Defrag.exe has the IO Priority set to Low.
I think this can help you in understanding the resources utilizations.