Quickstart guides get you a slow Synapse.
This is mostly a delta guide, unfortunately - you'll need to read all the other things to get this to work.
If you already have a Synapse server instance running, and you're going to switch from sqlite to postgres... Use Synapse's migration script. If you don't use the migration script Very Bad Things are supposed to happen.
/etc/matrix-synapse/ Contains .yaml, homeserver.yaml, *.log.conf, etc.