>The CTO of a startup I worked at hated dependencies. We worked with some 3D calculations (software for drones), and he was writing tens of mathematical functions himself.

What a terrible idea. Implementing mathematical functions is extremely hard to do well. And by well I mean "function properly at all". This isn't about speed, this is about the fact that if you haven't done actual research into what you are implementing, then your implementation is going to full of errors, many of them totally non obvious. Rolling your DIY numerics, without spending a lot of time on it is just asking for problems.