Why assume they're slow? Even if they are, you have to take into account the length of their legs. At a normal walk like what we saw in the trailer is going to eat up a lot of ground. Giant strides, depending on their size, could be two or three, even four, of our running steps. That would be pretty quick.
A Giant running? We might not be able to catch him or her to talk or do more dastardly deeds.
Sure, but it takes a long time for a train to stop. Top run speed is one thing, good maneuver speed is another. Getting behind a giant might actually be plausible if they take inertia into account during movement.
Anyways I think you should bite its ankles.
On a slightly serious note I don't want to see ankle smashing from the melee attacks. I can accept melee having a hard time vs giants since they have to set things up to get attacks to vital areas, but there should be high end creatures where melee types have the advantage.