Depends on what you're using the cache for. Postgres is definitely fast enough for reads of a K/V pair at a scale your master node can support for most use cases, but there are cases that you can't suit with postgres alone at-scale, e.g:

1. Needing a consistent (meaning reading off replicas is not fresh enough) data source for reads and writes that happens every request to very high-request site. Think sessions, api rate limiting.

2. Needing a fast store for denormalized data.