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?
Yep
In our databases we have extremely high disk I/O, I'm wondering if distributed transactions may be the reason behind it.
could be, but you have to send us more info about your setup, your configuration , especially your io settings, output of vmstats would also be helpful
