Mixed DRAM Runs At The Speed (Or Timings) Of The Slowest DIMM
Let’s say you have a DDR3-1600 CAS 9 DRAM module and you add one rated at 1866 CAS 9. One outcome is that the DRAM will go to the motherboard’s default of 1333 CAS 9 or 10 (or, many AMD motherboards default to 1066). Or, both will run at 1600 CAS 9 (or 10 or even 11) if DOCP, EOCP, XMP or AMP was enabled before you added the 1866 MT/s module.
But you can also manually set them to something else. Typically, in a scenario like this, I’d try 1866 at 10-10-10-27. Add a pinch of voltage to the DRAM (e.g., +0.005V). Depending on the results, you can also tune the MC voltage.