> Map is like a Go map[interface{}]interface{} but is safe for concurrent use by multiple goroutines without additional locking or coordination. Loads, stores, and deletes run in amortized constant time.

https://golang.org/pkg/sync/#Map

Also that for most uses a concurrent map is way overkill, and a thread-safe one is both costly and basically useless (hence the Java folks not keeping the thread-safety when migrating from Hashtable to HashMap).

On the other hand they're kinda shit given how awful non-builtin data structures are in Go, and how easy it is to "leak" maps between goroutines.