Dennis already explained why TIO isn't a great choice. The solutions I've seen people use are:
Cloud providers (e.g. Amazon Web Services' EC2)
Tell people what specs you'll buy, wait a week or so for a batch of answers, then rent one for a few hours to keep the costs tiny (we're talking pence unless you decide your challenge needs immense power). When you're done, delete the instance and move on with your life.
If you've never used AWS before you can even rent one small EC2 instance for free during the first year (but keep an eye on the CPU credits for T2 instances or your challenge will be unfair). I imagine most cloud providers (Google & Azure spring to mind) have a similar promotion.
Cheap computers (e.g. Raspberry Pi)
Buy a common dirt-cheap computer and run submissions on it whenever you want — again, tell people in advance what the hardware specs are.
You don't even need to have a screen / keyboard / mouse: just connect it to a network and SSH in (but don't forget the power lead!)
Both of these options set a level playing field and (theoretically) allow people to run their submissions in the target environment in advance. Both come with some cost, but not much for occasional challenges. Another bonus is that these are real world environments, so through your challenges people may learn something that helps them in their day jobs / uni courses.