Lukas Fitti writes in detail
on the pganalyze blog about the asynchronous I/O capability coming with the PostgreSQL 18 release.

Asynchronous I/O delivers the most noticeable gains in cloud
environments where storage is network-attached, such as Amazon EBS
volumes. In these setups, individual disk reads often take multiple
milliseconds, introducing substantial latency compared to local
SSDs.

With traditional synchronous I/O, each of these reads blocks query
execution until the data arrives, leading to idle CPU time and
degraded throughput. By contrast, asynchronous I/O allows Postgres
to issue multiple read requests in parallel and continue processing
while waiting for results. This reduces query latency and enables
much more efficient use of available I/O bandwidth and CPU cycles.

https://lwn.net/Articles/1020452/
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