I’ve been experiencing the same issues and found this post.
In reading the other post, it seems the issue is related to the VMware Paravitual SCSI adaptor.
As a test I changed the SCSI controller type from VMware Paravirtual to LSI Logic SAS. After this change the high response times seemed to be resolved. I have been copying data at over 600Mbps for an hour and response times are less than 7ms.
This seems to be an effective work-around in my case.