Raid Write performance issue

So I have a C236 based board and I have 4x Seagate 10TB drives in Raid 5 with the ~30TB volume formatted with ReFS.
But lately (a few months now) when i copy large files to it (IE 30GB+) after initially going fairly quick the transfer speed nose dives.

At first I thought it was a network issue, but the same happens when I copy locally from ether the OS drive or App drive (both are NVMe’s) to the raid volume.

Copies over the network to ether the OS or App drive remain fast, but any copy to the Raid volume nose dives in throughput.

At this point I think it’s ether something with the raid array or with the ReFS formatting.

@DarkLogix :
The reduction of the write speed may be caused by limited caching volume.

One part that’s odd is it didn’t have this issue at the start.
I used to be able to put 30-80GB files on it and it’d write it no problem.

The four drives are Seagate Exos 10TB drives
Model number ST10000NM0146

The system has 32GB of ram.

I have noticed that when the write speed drops that the files being written are listed as ram mapped files (via RamMap from SysInternals)