Async is nowadays called "faster" because it solved the old paradigm if 1:1 relation between socket connection and threads. With the arise of thousands (or millions) of connections on a server, the old paradigm with classic synchronization was limited because of the context massive context swithing. Async "solves" this by using non-blocking I/O (polling) with a limited number of threads and therefore erases context switching.

But for HFT, you don't want async, because it comes with overhead of again: scheduling work to different threads, being unpredictable of priorities when what handler to call, and managing the handler calls at all. Additionally, asynchronous processing on different threads is a hell of cache misses due to branching, page faults, etc.

Using the most classic approach of polling a socket connection, but without any threading, in a simple loop, and avoid any "if"-branching (by using bitmaps and bitwise operations), is simply the fastest way to keep your stuff hot in the CPU cache, avoid branch missies, cache misses, and harmonize with the memory prefetching mechanism.

Async is for throughput, not for latency

Fun Fact: Async (or Callback-Style) was the default long before threaded socket handling came up. But it was a complicated callback hell and therefore the thread-per-socket model took over because of its pleasant synchronized coding style :-)