Optimal Weight and Running Speed

Hobie Call ran a low 17 5 K with 30 lb vest. His weight is 142lb, I believe. He would have run a low 15 or maybe a little under without it. Rough approximation from that experiment could be t_orig * ((weight + extra_weight)/weight)^0.95, so 8:00 mile = 480 seconds for a 150 lb runner who picks up extra 25 lb becomes 480*((150+25)^0.95)=555 seconds or 9:15 per mile. If you want to do the computations, just substitute the parameter names (like t_orig or weight) with numbers and paste it into Google search box.

However in practice it all depends on the nature of the excessive weight. How is it distributed and how does it affect your biomechanics? Does its presence create a metabolic/blood flow/heart power/etc inefficiency of some kind?

I think it is a mistake to focus on weight alone. Excessive weight is only one of the many symptoms of the lack of fitness, and in some cases it may not be present. Sometimes increases in fitness can be accompanied by a weight gain, or a weight loss can happen along with the loss of fitness. One of my training partners dropped from 135 lb to 125 lb at first when he stopped training, then got back up to 145 lb, for example. Apparently, the body responded first by losing the muscle mass, and only then began to accumulate fat at a fast rate.

So short answer - if you lack the fitness and feel heavy, the fix is a good balance of training, eating, and sleeping. The fitness will improve, and the weight will arrive at its optimal point.