The easiest analogy is thinking of each port as a water pipe that can move a fixed amount of water.
if you have a pipe from comp1 to switch1 that can move 1gbit and a pipe from comp2 to switch1 that can move 1gbit, then the max speed bwteen the two computers is 1gbit.
The pipe from switch1 to router1 is 100mbit, so the max speed of 100mbit to the router is split between the two computers. not to say you have 100mbit internet.
Network packets are fairly efficient and it's all about weakest links.