The project will go at the speed of the slowest skill the members have.

So if you're faster than your co-founder, you'll feel that as slowing you down. But the real question is whether you'd be faster at that task than them.

E.g. if you rock the backend, and are forever waiting for the frontend to catch up, you'll feel slowed down. But that's only true if you're also faster at the frontend work than your co-founder, and able to maintain that speed while doing both frontend and backend.

There's a point at which having someone else do half the work makes the thing go faster even if they're not as fast as you.