At work we've been able to use mimalloc to great effect, using it as a drop-in replacement for the standard msvc allocator showed 3x runtime (!) improvements on multi-minute workloads. In terms of memory use we also saw improvements, but less dramatic.
Of course these numbers mean nothing without context (and there was much more heap allocation going on than needed), but getting that kind of performance improvement for (comparatively) such little effort really changed my view on memory and optimization.
Not the most user-friendly library though, perhaps that is a result of its selling point: 0 code changes needed.
HN discussion from 2019: https://news.ycombinator.com/item?id=20249743