The planned divert scenarios are just that... planned. An arbitrary speed is chosen, and for simplicity is turned into a distance based on still air. That distance is what is used on all your charts to draw the circles etc. It is usually not realistic to what you would do in real life, with most operators basing it on a very fast speed (to give bigger circles).
If, in real life, you have an ETOPS failure then you fly whatever speed YOU deem best (which is likely to be FMC speed). You do not have to be on the ground within 180mins.