Well, if you are copying small files, this might be normal. I have seen some test comparing hdd vs ssd where transfer speed while copying small files dropped on hdd even under 1MB/s (depends on fragmentation, file-size, etc). Tests were done in real environment (no VM) and iirc the hard-drive was "raptor" (one of the fastests hdd).
Just for comparison, try to copy single big file. But if you are dealing frequently with small files, maybe it is time to put vm-datastore on ssd...