Another problem, even in functional code, is that data dependencies tend to be linear. It's difficult to write code that can be automatically parallelized.
"The group has written sev- eral million lines of code, including: core libraries (includ- ing collections with polymorphism over element permis- sions and data-parallel operations when safe), a webserver, a high level optimizing compiler, and an MPEG decoder. These and other applications written in the source language are performance-competitive with established implementa- tions on standard benchmarks; we mention this not because our language design is focused on performance, but merely to point out that heavy use of reference immutability, includ- ing removing mutable static/global state, has not come at the cost of performance in the experience of the Microsoft team."
"we mention this not because our language design is focused on performance, but merely to point out that heavy use of reference immutability, ...has not come at the cost of performance in the experience of the Microsoft team."