If asked for feedback, you tell their manager that the employee's performance is subpar and you provide very specific examples of that. Leave it up to that person to manage their performance -- that's what they are paid to do.
In terms of you working with this "not so smart" person, suck it up and remember that it's just a job.
The company might fire the low performer and hire someone better. And depending on the new hire you can end up being the low performer…
An alternative is to mentor the low performer and become their manager eventually; of course this can backfire too, especially if you’re not interested in the management track.
My point is, working with a not so smart coworker isn’t that bad, at least comparatively.
There's typically a substantial difference between performing at a level below the highest-rated team member and performing at a level below team expectations.
If asked for feedback, you tell their manager that the employee's performance is subpar and you provide very specific examples of that. Leave it up to that person to manage their performance -- that's what they are paid to do.
In terms of you working with this "not so smart" person, suck it up and remember that it's just a job.