Optimize queries that need to be 100% ACID compliant but can tolerate latency in persistence
Your question seems to be logically inconsistent, as tolerating a latency in persistence and "Actually I can afford to lose transactions that are in memory only" by definition violates the "D" in "ACID".

If you really want to be just "ACI" compliant, you can turn off "synchronous_commit".

Note that this means you could report to the traders that their trade executed, but then later find out that oops, it actually didn't.  It will still be atomic and consistent, as both sides of the trade and both changes in cash balances will have disappeared together.  But this could still be a bad customer experience.

If you decide you can't run with "synchronous_commit" off, you should at least do the test with it turned off.  The outcome of the test will indicate how you should proceed for further optimization.