I'm not sure what the answer to this question is in general, but from my perspective (was a CS major in the mid 1990's) there were definitely parts of this that

I'm not sure what the answer to this question is in general, but from my perspective (was a CS major in the mid 1990's) there were definitely parts of this that were hard (by my standards) by then. All of the computational complexity stuff (P vs NP, etc.) feels akin to black magic to me. Especially so when you get into that whole "complexity zoo" thing where there are 10 bazillion different complexity classes. To this day I have no idea what most of them mean and have to visit the corresponding Wikipedia page and read up on any ones that I encounter.