HP dl360p G8 writes/latency with SSDs in raid very poor

RAID-6 is a bottleneck because all writes are limited by the performance of a single drive and have a write penalty on the random or even sequential block though. Additionally, the idea to test the performance of the datastore by simply copy and paste is not an accurate approach of benchmarking. Deploy a VM on top of the datastore with eager zeroed provisioned VMDK and test the performance using fio or diskspd. Moreover, while copying from HDD to SSD the operation is read-write which means that data is read from the HDD with the maximum performance.

Also, check this article from Microsoft - https://blogs.technet.microsoft.com/josebda/2014/08/18/using-file-copy-to-measure-storage-performance-why-its-not-a-good-idea-and-what-you-should-do-instead/