I've recently started using Folly's F14 hash map implementation (specifically, F14ValueMap--my keys and values are relatively compact) and it's the fastest unordered associative container I've yet found. This is for an application that repeatedly adds to and removes from the map. In particular, it's faster than a custom solution using open addressing that I had written that is pretty much optimal for this particular application (for an open addressing implementation, that is).
I do wish it could be used header-only out of the box, but still a great library.
I do wish it could be used header-only out of the box, but still a great library.