Maxwell’s equations find that the fastest speed is the speed of light. The speed at which electromagnetic waves are carried; The fastest way to get from one place to another in the universe. This is the fastest and unchanging speed as you look at it from a standstill and follow it with a beam of light. In addition, it is the speed at which only particles without mass can run.
The presence of mass slows down the obstruction. The universal speed limit is not infinity just because there is mass. Instead of infinity, the universal speed limit is the speed of light. And it’s a velocity constant of causality. The main idea of E = mc² is that you can change the amount of energy from a small mass to a large amount. c is a parameter that sets the causality speed limit.