I got approached by an external recruiter, then I had a first call with the lead engineer, I got a take home task and I had another 1h 30 min call with the lead engineer and the project manager

I had 3 rounds of phone interviews. During the first round I had a very nice conversation with their manager. He told me a lot about what Glooko does, how blood sugar level measuring devices function, etc, etc. During the second round I easily solved some programming problems (I chose C++), and had seemingly impressed the interviewer. During the third round of phone interviews another interviewer asked questions about how to optimize the Python code. I told him about multi-threading, muti-processing, cython, etc. I also told him that Python code really shouldn't be optimized. If you find yourself in a pressing need to optimize the python code because your system performance is low this likely means that the Python code needs to be re-written in a more performant language like C++. He didn't seem to like my last answer. There are a lot of people who are of persuasion that Python is "good enough" for almost everything, and one just needs to optimize things better. This is certainly not the case, but many people believe this, including this last person who interviewed me on the phone. Soon after I got the rejection e-mail "your background looks impressive, but we decided to go with a candidate with skills that better suit our company". Yeah, right, and I know what that "skill" is: Python is the main language + the belief that Python is good enough for everything and there's no need for other languages. If your skill-set spans beyond Python and you understand other languages well you probably ain't a match. Don't focus on this, at least.