Depends on your application. In most cases your "write heavy" is going to be isolated to 1 or 2 tables worth of data (or equivalent) and for those cases some type of NoSQL solution can be a really good option. Especially since you can use a PG foreign data wrapper to allow PG to run queries against that information too.
If it's system wide and you need PostgreSQL itself able to handle it, using PG's async features is one potential option but setting up and managing a Postgres-XC cluster would be the next best. XC allows scale-out for writes. If you're at a company with a budget for that type of thing I think I remember reading that EnterpriseDB (the PG company) is offering first class support for PG XC.
In most cases though, I find that the write heavy parts of a system are so isolated that diverting them to a simple NoSQL solution tends to be easiest (Mongo, Couchbase, DynamoDB from AWS, etc).
If it's system wide and you need PostgreSQL itself able to handle it, using PG's async features is one potential option but setting up and managing a Postgres-XC cluster would be the next best. XC allows scale-out for writes. If you're at a company with a budget for that type of thing I think I remember reading that EnterpriseDB (the PG company) is offering first class support for PG XC.
In most cases though, I find that the write heavy parts of a system are so isolated that diverting them to a simple NoSQL solution tends to be easiest (Mongo, Couchbase, DynamoDB from AWS, etc).