Yes. Putting files on to the server / pool is fine as I'm using a SSD to buffer incoming files. It's getting duplicated files back off the pool that is slow. I have since discovered unduplicated files are read normally whether read striping is on or off.
I have been experimenting with local file transfers from the server's pool to it's own C:\ drive.
Read striping enabled does not seem to slow down single large file transfers but it will only use one drive so the maximum transfer rate is the maximum of whatever disk it is coming from. Read Striping bar is 100% dark blue. (My interpretation of read striping is that it will transfer faster than the maximum drive speed as it can read from multiple sources but I am quickly figuring out that read striping is more for better access to lots of files at once.)
Multiple small files are slowed down slightly. Photographs transferring at ~72MB/sec without striping, ~55MB/Sec with it enabled. Multiple drives being used. Read Striping Bar all 3 colours but mostly dark blue.
Network transfers from the server to my PC:
Read striping enabled causes single large files to transfer erratically. When the bar is 100% dark blue, it's only using one drive and transfers at 110MB/sec (network maximum) but when it starts going light blue up to 50% and using multiple drives, it slows right down to 45MB/sec.
I have tried swapping out the Dell PERC H310 raid card for a HP H220 host bus card to no appreciable effect.
So it seems to be there's a bottleneck appearing somewhere when the system is trying to use more than one hard drive at a time. I'll see if there's any firmware and / or driver updates for the PERC card and SAS backplane. Is there anything in Windows itself that may be causing this?
Edit: There was an Intel chipset update. It improved single file transfers over the network although the transfer speeds are still erratic and slow when using multiple disks. But overall a bit better than before as it's not using multiple disks as much. Multiple small file copies with read striping enabled are now down to 10MB/sec using multiple drives or 80 - 90MB/sec with read striping disabled.
Further experiments:
I duplicated a folder with big files 3 times so it would go on to the SSD as well. Copying from that is fine with read striping enabled but it seems to be because it's mostly using the SSD. HDDs sometimes get used but doesn't cause any slow down. Remember the SSD is a M.2 drive on a PCI-Ex adapter card so it's not using the same interface as the HDDs.