DD test speed dips over time

I am setting up a new system and wanted to burn-in the HDDs before use. I ran the following command on each drive in parallel.

dd if=/dev/sdx of=/dev/null bs=1M status=progress

Is it normal for the speed to dip over time? (The dips at the start is me cancelling the test and figuring things out).
It started at 249MiB/s now at 173MiB/s

The drives are: ST12000NT001

This might be the most normal thing but I am not knowledgeable enough to know if it is or not.
Thanks

This is what it looks like:
https://imgur.com/a/elfWme0