As @wildpig1234 mentions, the more you have, the slower it goes.I have 250gb ram and even though they are 1600mhz, bios will say they are operating 1333mhz and if I add more, it will even go down to 800mhz. And the RAMs are running in independent mode. I am running X9DRI-LNF4. Are you getting full RAM speed?
There's chart's in the Manual for your motherboard that will tell you how slow. Find the chart for your CPU (v1 or v2) and RAM (RDIMM or LRDIMM) types, then find the line corresponding to the "Ranks Per DIMM and Data Width" for your specific RAM. From there, it will list the maximum speed(s) based on how many memory slots per channel you have populated.