Hi @mattiascalas,
check out the encoders for this:
https://github.com/GIScience/openrouteservice/blob/master/openrouteservice/src/main/java/org/heigit/ors/routing/graphhopper/extensions/flagencoders/FootFlagEncoder.java#L45-L48
Also see:
As for why, that’s a mixture of trial and error for reasonable results, common sense and actual trial runs. But nothing documented.
As you could factor in a lot of stuff here (height, gender, age… just the first page i found 
) i think the 5 km/h is a fitting general estimate.
Best regards