hard drive - File transfer between two HDD seems to slow down other activity on main SSD

I experienced some unexpected behavior trying to install a large program (60GB) on to my primary SSD at the same time as I was transferring a large file (2TB) between two HDDs: the installer slowed down to a crawl.

Additional Observations:

• As soon as the file transfer finished, the installer resumed at normal speed. The two situations appear to be related


• The source and destination of the installer are the same, that is to say that the installer was located on the SSD and was installing to the SSD. The source or destination disks of each file operation are different from one another.


• The large file transfer between HDDs was completed at a constant 195MB/s. This is unusual: in the past, transferring large files like this on Windows would start at 195MB/s then slow down considerably to about 80-90MB/s


• In Task Manager, I examined drive activity: the HDDs had a lot of activity but the SSD had no activity.


• I didn't monitor RAM usage closely, but I did note that there was no unusually large usage reported.

System specs:

• Windows 10 Pro 64-bit v1909


• 32GB DDR4 Ram


• Motherboard is GIGABYTE Z270X-UD5


• Primary drive is a M.2 SATA SSD in M.2 slot (M2M_32G)


• Two other HDD attached to the computer on SATA ports

Question

What could cause this? And how can I remove or alleviate that bottleneck? My gut instinct is to suspect some hidden buffering system going on behind the scenes that is bottle necking this transfer. Especially if Windows is buffering to the SSD behind the scenes? If that's the case, is there a way to instruct Windows not to do this? This behavior also seems to be relatively recent.