One way to ensure whether you're hitting maximum storage access speeds is to use another medium, such as a usb 3.0 drive, or transferring between drives on the machine. Since your Windows 10 device only has the "one" drive RAID 0, internal transfer may not be an option hard drive wise, but if you have speedy external storage that you can plug in to, that may help you determine whether your drives are operating slowly. RAID 0 should easily allow you faster than 40-120mb/s, even if they're slow, though. The double Western Digital Reds though...
From Tech Radar's review of the drive:
Crystal Disk Mark reported sequential read speeds of 173 MB/s and write speeds of 165 MB/s. This is slightly lower than the claimed 175 MB/s speeds, but equivalent to HGST's He6 6TB disk and less than Seagate's Enterprise Capacity 6TB (223 MB/s).
This may be your issue. Since they're in RAID 1, together, you should be expecting speeds much faster than the ~150-200 when reading since RAID 1 acts like RAID 0 when reading, but when writing it will write slowly. If you want to test it, I would try hooking up external storage to your server also, and see if speed improves significantly. Red (NAS) drives are generally pretty slow because they're built to be reliable more than anything else. They are likely the cause of the slowdown that you're seeing.
As for actually answering your question of speeding it up: http://www.dell.com/downloads/global/power/ps3q07-20070368-Olivarez-OE.pdf has some good tips but you'll want to look at ensuring that raid controllers are updated or looked at in case you can find better, more optimized, drivers, or consider switching to RAID 5 if you have the income, as that could help your performance as well. That would result in purchasing 1 or 2 additional drives though. I hope this helps.