I understand that tick method is called 60 times per second in DragonRuby. But what happens when it cannot do it (system resources busy or simply too many calculations)? Does it get called less frequently or is it called as often as possible, but the game simply slows down? What happens to tick count?

It's possible to configure more frequently tick calls?

The game will run slower and will go back to 60hz as soon as resources are available.