Why do they have to be slow. if robot or mechs keep developing as fast as they do now they will be faster then a normal car or an heavy thank.
If by a "Mech" you mean the popular rpg image of a two-legged battle robot stuffed with all kinds of weapons then you
won't see them in the future. No matter how your technology advances.
The only reason to build mechs is because they look "cool".
The real world military won't build them because whatever the mech is capable of doing a weapon platform chassis can do better, because quite obviously whatever you can mount on a mech you can also mount on a platform.
A vertical standing mech is a bigger target and more unstable than a horizontal platform. Usually you can even mount more stuff on a platform. Also if you need a battle machine capable of going to places where a tracked vehicle can't then the obvious solution is to fly. In Afganistan you use choppers.
If for some reason flying is out and you really want a land battle vehicle on a rugged terrain then you don't build a
vertical mech. What you want is a
horizontal mech. E.g. a battle vehicle that moves somewhat like a cockroach. (BTW: that's even a real world thing. There are experimental very small explorer robos that mimick cockroaches in their moving and who mount a video camera.)
IMO the only thing dumber than a military building battle mechs is a military building Imperial AT-ATs especially when even a farmboy can afford an anti-grav device...
However, this doesn't mean you won't see battle robots in the future fields. They just won't be two-legged. Instead you probably will have tanks, fighters, ships, and all kind of vehicles but steered with a computer AI instead of a human.
Besides there already is one: The Predator