As the OP stated so too have I bough a second set of wheels and tires for the trails. However instead of Marathons for the road, I've gone 32mm Pirelli Cinturatos tubeless (60psi) which are know to be a reliable puncture resistant tire with a road bias. They will handle smooth gravel and light off roading no problem. I have them in different sizes across the fleet and I like them a lot. For the trails, 40mm Pirelli gravel M also tubeless (30psi) which may have been over the top in hindsight. I went for robust over speed and can't say I regret the way it deals with rough trails. The cassette is the same on each wheel to minimise any issues with changing wheels. Even with a minor rear brake adjustment with the wheel swap its been a good strategy.

In terms of speed there is no way to say I suspect. Trails tend to have highly variable surfaces and depending on how loose or rocky the trail is survival may dictate the speed at which one progresses compared to how "fast" a tire might be. Weight I believe is a factor since pushing a heavier tire all day is likely to require more effort than a lighter tire. Reviewers who allegedly know about such things have said the Pirelli gravel H is ridiculously fast wherever it is they ride. Its possible the rougher trails where I ride would destroy it and I prefer to keep the air in the tire if possible. I value air staying in the tire and I've not had a flat for a long time, I'll go with the more robust tire every time. My wife has Marathon tires on her bike and they are fine on smooth trails.

I don't believe there is an answer to the question because everyone's experience differs. This is just mine for what its worth.