What happens when you mix RAM sticks with different speeds and timings?

The system runs at the lower speed (2666MHz) and the looser timings (CL16) based on the slowest stick’s JEDEC profile. The 3200 stick downclocks to 2666. Dual-channel mode works only for the first 8GB; the rest runs single-channel. It works fine for everyday use, but you’re leaving performance on the table.