Still a mismatch. You either want 2x8 or 4x8 (all running at identical speeds). I know it's tempting to use what you have to give yourself more RAM, but the "more is better" only holds true if all sticks are functionally identical. Your computer will run them in single channel mode and it will lower the speed of whichever kit is faster to match the slower one, so you are getting hit with a double performance drop.

Pick up a 2x8 kit of the same kind of RAM (same capacity, same timings), or get a faster kit to replace your current ones.