memory speed decrease as more RAM is added ?

hi folks,

I have heard but cannot confirm that the more memory one adds to a machine, the slower the RAM becomes.

Is there a threshold from which the memory speed decreases?  Does it relate to specific memory, specific CPU's or motherboards or is a general issue, commonly known?

Basically I have a Dell T7910, currently with 128gb RAM (2133mhz speed) and would need to increase to 256gb or more and I'm concerned about the RAM speed decrease.

Any pointers are much appreciated,

thank you,

Cos