Transfer rate reduced to 50% after 2 hours

I'm copying a bunch of data over to my new Synology DS220j.

Write speeds looked good for the first two hours with full gigabit speeds, but you can see the drop off in transfer speed on the graph.

Could it be throttling because of heat? Poking around Control Panel I noticed the fan was set to "quiet mode".

What can I do to troubleshoot at this point? New to Synology.

Edit: added graph

https://i.imgur.com/2CYU6yg.jpg