So I
bought acer nitro 5 gaming laptop just recently and it comes with 2*4GB RAM
Sticks. CPU Z confirms the RAM sticks are kingston DDR4 2666 and really runs at
2666MHz (you can see attached screenshot).

Since the
preinstalled RAM is just 8GB, I upgraded it with a pair of this (to make it 16GB):
Ballistix Sport LT 8GB Single DDR4 2666 MT/s (PC4-21300) SR x8 SODIMM
260-Pin - BLS8G4S26BFSDK (Gray).
However the upgrade RAM sticks, as confirmed by CPU Z, run at only 2400MHz, not
the advertised 2666MHz (see attached image).

I now wonder why the preinstalled ddr4 2666 ram runs at 2666, while the
upgrade ddr4 2666 ram runs only at 2400 on exactly the same machine.
Any workaround to attain the 2666 speed of my upgrade RAM sticks? I could not
see XMP option on my laptop BIOS!