A combination of capacitance that has to be charged, crosstalk and characteristic impedance discontinuities. The signals move between the cpu and the ram at ab

A combination of capacitance that has to be charged, crosstalk and characteristic impedance discontinuities. The signals move between the cpu and the ram at about one nanosecond per foot. A nanosecond is one full cycle of a 1GHz clock. The characteristic impedance of the connection is set by the physical size of the pcb track and the dielectric constant of the pcb material. As the signals move between the cpu and ram at nearly the speed of light if they hit an impedance discontinuity some of the energy is reflected back. That messes up the signal. Making a connector with hundreds of pins that works reliably and does not cause a significant impedance discontinuity is hard. The lower the impedance of the connection, the more power is required to drive data through it. A bit error rate of one in a billion would corrupt data or cause a software crash daily so signal integrity is a hard problem.