If you're confident about it, I'd assemble a few details about the run and try and articulate briefly why you think it's cheated. If the mod isn't contactable I'd ask other runners about it, if there genuinely aren't any available either the support hub could well be the way to go, provided you're also a runner of the game. In the support hub you're able to file reports against users for that type of thing but it's imperative to be clear to the site mods who won't be familiar with the game.
The thing with cheated runs is it's a very serious accusation to levy against someone, if there's a chance it's a mistake or some exploit you're not aware of, try and rule them out.

Out of interest what game is it?

Goosebumps: Attack of the Mutant doesn't work properly if another window is open. Does anyone have an idea of how I can capture my runs? Usually I'd just use OBS.

I can't beat the top run for a game because his run is loading faster. Can I submit and ask the mods.to.remove loading time?

Speedrunning

Need help setting up your stream, or recording your runs? This is the place to ask.