I wish I grew up on metric but I didn't so now i'm stuck scratching my head when I think about the metric system. Then I just convert and carry on without any more thought.
Metric system isn't hard at all. You've got your base term like meter or gram. The rest of the terms are some positive or negative multiple of 10. Schools spend too much time trying to get you to learn them all. All you need to know is milli (1/1000), centi(1/100), and kilo(1000) mostly.
What is hard is conceptualizing those values when you grew up with feet and pounds.
What the hell is 30 centimeters? Its a foot, but if it isn't something nice and even like that, you are back to doing a conversion in your head.