...That is the realistic "install Postgres and go" baseline. RDS ran on its default parameter group, which AWS tunes for the instance size. Hostim runs its own managed tuning. In other words, the two managed options are tuned out of the box and the self-hosted default is not, and that difference is part of what you are comparing.
So... useless comparison vs Hetzner? Akin to comparing two laptops on a compile job, but leaving one in battery saver mode and then concluding that its 2x slower.