Small example to show that performance can be great: Phoenix (the Rails-comparable web framework for Elixir) defaults to listing microseconds instead of milliseconds for response times.
What I really meant was, as a backend engineer, I frequently deal with optimizations on too many object allocations and long running/too frequent GC cycles even without immutability built into the language.
On the Rust front, the problem is in small memory allocations, fragmented memory and then more calls to kernel to alloc.