No, a synchronous procedure is a procedure whose minimum completion time depends solely on the speed of the CPU and memory system (and lock contention, in some models), while an asynchronous procedure is a procedure whose completion time also depends on external events.
In other words, synchronous procedures are essentially locally CPU-bound, while asynchronous procedures are bound by I/O, network or IO/network/CPU of remote servers.
Async functions are the most efficient way to support asynchronous procedures, while non-blocking non-async functions are the most efficient way to support synchronous procedures.
There are also adapters to turn synchronous function into asynchronous ones, and to turn asynchronous functions into blocking non-async functions, although they should only be used when unavoidable, especiallly the latter.
In other words, synchronous procedures are essentially locally CPU-bound, while asynchronous procedures are bound by I/O, network or IO/network/CPU of remote servers.
Async functions are the most efficient way to support asynchronous procedures, while non-blocking non-async functions are the most efficient way to support synchronous procedures.
There are also adapters to turn synchronous function into asynchronous ones, and to turn asynchronous functions into blocking non-async functions, although they should only be used when unavoidable, especiallly the latter.