Marathon time loss due to tights?

Because in a race you generate a lot heat and have additional adrenaline flowing. If it's above 20 degrees Fahrenheit at the start of a race you aren't going to see elites wearing tights - especially males.

Any "time loss" would be due to overheating - which proves that you don't need them in the first place.

It does not sound pleasant, but a good slathering of petroleum jelly can help keep your legs warm for a while. I do not think that tights slow you down that much. The cold may slow you down more than wearing tights as your heart has to work harder to pump more blood to your extremities.

I guess I was thinking in terms of weight, kinda like a light weight racing shoe vs a heavy trainer.

I guess tights don't weigh that much, but they are on the legs which are a swinging like your feet.

No time loss. If you think it is going to be too cold for you then wear tights. Someone on a message board can't make that determination for you.

Worst think that happens wearing tights is that you get too warm. Worst thing for wearing shorts is you get too cold. Either way you will learn how to dress for a future race in similar temps.

Personally I'd go with the tights.

This is backwards of what most runners like which is shorts over tights but I would go with tights over shorts. If the tights get too warm, you can just slip them off and still have your shorts underneath.

Rather than full tights, what about knee length ones? Or capris?

As for weight, tights are not like shoes. Since the shoes are end of your feet (pendulum) the weight is going to be more of an issue than tights that are spread over a greater distance and closer to the center of mass.

As for elites, my guess is that it rarely is cold enough for them. There is some evidence that getting below 32F limits marathon performance. A google search should come up with some papers.