No, the performance problem didn’t exist. The engineer in question was really smart in many other areas, but “engineering” per se wasn’t his strength. Things are fast-and-loose in startup world.
Early on I had a senior guy mentoring me on a project involving a tool called PowerBuilder. He chose a design that didn't fit the problem-space well but it fit the "PowerBuilder Best Practices" so he implemented it. The performance was abominable and he should have known it would be: he too was a smart guy. But he had a hard time seeing "big picture" design.