Why does the Internet connection speed drop if one of the computers connected via the Asus RT-G32 router is turned off?

Question

Computer networks

Given: Two computers are connected to an Asus RT-G32 router. The Internet port of the router is connected to the Rostelecom provider, a standard PPPoE connection with authorization. Checking speed via speedtes.net Basically there are no other specific settings.
Problem: By experience and tests, I found out that: if one of the computers is turned off, then the speed drops from 25 Mbps to about 3 Mbps. If the second computer is turned on, then the speed increases back to 25. Physical switching of computers to different ports of the router does not affect the result.
Question: how can this be?

Answer 1

Some kind of mysticism is described, or you are missing some important details. Contact the provider's technical support to send a wizard to diagnose the problem.