>But the big question is, is it really that much slower?

So, there's a pretty good shibboleth/knowledge smell you can use to detect whether somebody's done systems programming and knows what they're talking about or not.

Whether they think relying completely on the heap is a "small" cost or not.

Try writing some code in a real-time constrained environment where non-determinism is unacceptable. See how far malloc and Boehm gets you.