I disagree. While non-vectorized operations are slow in Matlab, this is the first thing you'll find with a quick Google search. Every language has some performance pitfalls (ok, Matlab has many). Knowing the basics of how to write performant programs in your language of choice is a user problem.

I've seen plenty of non-performant C code. And that's one of the most performant languages you can code in, if you know what you're doing.

Yes, but the researcher who can’t really code isn’t likely to go and learn Julia. A nudge is often better than pulling the rug out from under them.