It's not a matter of performance, it's a matter of being able to
detect collisions on the line. Ethernet does collision detection by
sending a packet out on the line and then listening if anyone else on
the line is also trying to send something. If they are, they each
wait a random amount of time and try again. The amount of time it
waits is dependent on knowing what the maximum cable length is,
because it needs to wait for the packet to reach the end of the cable.
If it waits that amount of time without hearing anything, then the
entire cable must have been clear when it sent it.
If you try to use a cable longer than the maximum length, then you
risk throwing off the timing of the protocol.