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.
What would be the point of reference to know that you are 'blocking' when you are alone?
If you remove all blockers from your path and still remain blocked, it's a good indication that you shouldn't ever place yourself in a position where people are going to notice.
That is, if you're genuinely good at building things, you're going to build your thing quickly. There's nobody to drag you down, no slow backend guy blocking you from implementing the frontend or vice versa. Or maybe you find out it was you doing the blocking all along, which is also handy to know.