Distributed transactions and asynchronous commit

[Xenofon Papadopoulos <xpapad@xxxxxxxxx>]

In the asynchronous commit documentation, it says:

The commands supporting two-phase commit, such as PREPARE TRANSACTION, are also always synchronous

Does this mean that all queries that are part of a distributed transaction are synchronous?

In our databases we have extremely high disk I/O, I'm wondering if distributed transactions may be the reason behind it.

Thanks