looks like they're just multiplying two 100x100 matrices, once? (maybe I'm reading it wrong?) in Julia, runtime would be dominated by compilation + startup time.

A fair comparison with C++ would be to at least include the compilation/linking time into the time reported.

Ditto for Java or any JVM language (you'd have JVM startup cost but that doesn't count the compilation time for bytecode).

Generally, for stuff (scientific computing benchmarks) like this you want to run a lot of computation precisely to avoid stuff like this (i.e you want to fairly allow the cost of compilation & startup amortize)