So first, there are two types of velocity when it comes to signals on a transmission line which is what metal lines on chip are: phase velocity and group velocity.
Phase velocity is close to the speed of light. It can be reduced by dielectric materials parameters so it's often lower than c.
Group velocity is often much slower. Information transfer can never be faster the group velocity, rather than the phase velocity. So when it comes to "on chip communication" it is group velocity that matters. Group velocity depends on many factor of transmission line design such as line thickness, line width, dielectric thickness, etc.
Phase velocity is close to the speed of light. It can be reduced by dielectric materials parameters so it's often lower than c.
https://en.wikipedia.org/wiki/Phase_velocity
Group velocity is often much slower. Information transfer can never be faster the group velocity, rather than the phase velocity. So when it comes to "on chip communication" it is group velocity that matters. Group velocity depends on many factor of transmission line design such as line thickness, line width, dielectric thickness, etc.
https://en.wikipedia.org/wiki/Group_velocity