Just because he misses his own deadlines doesn't prove that he is slow or a bad developer. Especially in the beginning, a lot of unforeseen problems can occur during development. Very few developers (if any) get the estimates right. Perhaps he also decides to pursue specific problems (ie if you are close to the deadline, you could either decide to release shoddy code, or miss the deadline - would you prefer shoddy code?).
Also, about the intern: the intern wants 9 to 5, your technical co-founder wants 12 to 20, and you are siding with your intern? Priorities? Get another intern who also prefers 12 to 20, or can get stuff done on his own.
Also, about the intern: the intern wants 9 to 5, your technical co-founder wants 12 to 20, and you are siding with your intern? Priorities? Get another intern who also prefers 12 to 20, or can get stuff done on his own.