This is a tricky problem indeed. Lin Manuel Miranda expressed a similar worry with writing Hamilton, and his conclusion was that it was about time; it took him years to write Hamilton, but Alexander Hamilton himself could have written it in months.

As the author, you have a few other advantages: 

1) You know what knowledge is available in your story’s world, including made-up knowledge, and can choose to make as much of it available to your smart character as need be.

2) You have the space and privacy to construct an entire flow-chart of things in relation to each other, and chart a path between point A and point B, and then have your character make the connection in a snap.

3) You get to write the dialogue for all the characters playing opposite your smart guy, and thus can set them up for as many snappy one-liners as you please.

But, when considering how to write a smart character, it might help to slow down and take some time to analyze what you think smart  means – because that’s not a simple question by any means! 

Smart  as in, lots of book-learning?

Smart  as in, can solve puzzles quickly?

Smart  as in, can make connections quickly?

Smart  as in, speaks in witty and snappy dialogue?

Smart  as in, can memorize new information quickly?

Smart  as in, has a perfect or eiditic memory, and can remember things indefinitely?

Smart  as in, can make leaps of intuition, innovation or invention that other characters do not?

Smart  as in, highly observant of their surroundings, and can find relevance in small details that others miss?

Smart  as in, has an innate talent (such as perfect pitch) which allows them to perform well at specific tasks?

Smart  as in, can see through others’ attempts to deceive or manipulate them, and know how to resist it?

Because there are a whole different host of ways to be smart! And being good at one or more of these tasks doesn’t necessarily make you good at all of them! (Although some of these are related talents/skills, and developing one likely makes your character good at another of these.) In fact, one of the easiest pitfalls to fall into is having one character who is good at literally everything.

If you want to write a smart character, I recommend taking some time to break down what things you think they’re smart about,  and how they became  smart in those areas – did they spend years studying? Did they spend time to practice and develop a particular skill? Did something about their background or circumstance encourage them to develop this skill to a high pitch? Is this an innate, maybe supernatural talent? (Try not to lean on that last one too much.) 

And to finish this up, I would beg that people avoid leaning on the narrative shortcut of trying to tell the audience that your character is smart by having them be Really Super Good At 1) chess or 2) solving rubik’s cubes. I’ve gotten so  tired of that.