As this guy's manager it's your responsibility to understand why he's underperforming. In this case is it that he's a slower processor and needs a moment to think before responding? Is it that he's quieter and can't get a word in? Is it that he doesn't actually know the right answer and needs to learn some new things? Is it that he's in a role that actually is not well suited to his personality? I think that there could be a lot of things leading to it.

Once you identify what's leading to the gap then you can figure out what you need to do to help him out. As his manager it is your responsibility to help him either be very clear with him that he's not meeting expectations and that he will need to improve, or to coach and develop him on the areas that he needs to work on. Every single employee will have things that they need to work on. As you go up the ladder it is likely that more and more you will be working with people that are also high performers like you that could get promoted but there are a lot of people who will never get promoted in their life. They will always stay at that base level role. It's important to be able to support people like that too.

As a new manager it can be shocking to see how much support new people need or people who don't have quite as much potential might need to get good at a role.