There is the fat Shienaran in TGH that is described as "every bit as slow as you would think due to his morbid obesity."

There is the fat white cloak that Perrin kills in TDR, thinking to himself that "the fat white cloak was even easier to kill than he had anticipated... and he had anticipated a pretty easy kill because he was so fat that he surely couldn't move very fast."

Then there is the fat guy that Fain kills in Tar Valon, noting that "he could hear a man's labored breathing around the corner. It was the labored breathing of a man at rest. Only super fat people have trouble breathing when they are at rest. This one will be so slow that I will have no trouble at all killing him." After finishing the job fain notes, "just as slow as I thought."

So as you can see from these several examples, not every fat person in what is deceptively light on their feet.