I already have a RAM module which came with the actual laptop. This was an 8GB module with clock speed of 2666. I purchased one more of the same speed and capacit. This runs in dual channel mode but brings down the memory speed to 2400.
Here are some screenshots from CPU-Z.
What am I doing wrong that the speed reduces? Are the RAMs not compatible?
The problem is that your memory has different timings for the memory modules.
One module can work at a higher frequency with a higher CAS latency, while the other module can only work at that CAS latency at a lower frequency. One module only supports CAS 17 at 1333 while the other works at CAS 19 at 1333MHz. In order to operate both modules at CAS 17 one module is needing to drop its clock speed.
The result is that the memory controller in your CPU effectively negotiates down the frequency until it finds a set of memory latencies that will work together. In your case this point is apparently 1200MHz (2400 DDR).
This is why people recommend getting identical sticks, preferably from the same batch to make sure that you don't get newer modules with different timings. Memory manufacturers can change specifications at any time without warning and you may end up with a "better" nearly identical stick that causes this kind of memory incompatibility.
Some references for mixing CAS latencies
Many people actively have trouble with different modules.
In some cases, and from experience, buying higher frequency memory modules can mean better compatibility with the top speeds of lower frequency modules, as they have more potential for overlap in latencies.
