I received the official statement from Dell support. They aknowledged they derated the system specifications from original 4267MHz to 3733MHz on 8GB and 16GB models with BIOS version 1.13.0.  From their point of view it is OK to derate the system and decrease performance after it was shipped to customers. What a shame.
Hello. When my Latitude 7420 came in with BIOS version 1.8.2, the memory frequency was at 4267 MHz. After I updated to latest BIOS version 1.20.0, memory frequency was reduced to 3733 MHz. This is 15 % drop and has noticeable performance impact on the GPU. The used memory chips HCNNNCPMMLXR-NEE from SK Hynix are specified to operate at 4266 MHz according to the manufacturer's website so there is no reason to decrease the frequency - [URL] Will the future BIOS version have the option to return the memory frequency back to 4267 MHz? Thank you.
Latitude 7420 memory frequency decreased by BIOS update