On the Bench today GaK_45 takes a look at the impact of mixing ram speeds

So is mixing and matching DDR4 RAM a ‘good idea’? Well that depends on how you define ‘good’. Is it an optimal solution? No. Will it work… probably yes. Will it be a simple endeavor, one that is as easy as using four paired sticks of RAM? No. It’s not impossible but you will have to be proactive and take steps to ensure that they work

http://www.realhardwarereviews.com/mixing-ram/

Great article, this was very informative. I don't think people consider all that was mentioned when they contemplate mixing RAM.

Thanks for the article. Always nice to see how various components change performance.