As a data point, we're using Redash (https://github.com/getredash/redash/), an alternative to Metabase, in a VM on Digital Ocean. It's a US$15 VM (2 cpu, 2GB ram) and seems to be fine. That's using a PostgreSQL instance running on the same VM, and nothing seems to be unhappy.
The graphs it generates are used both publicly (auto-updating):
... and we have a bunch more private graphs and dashboards for metrics.
Everything is close to instant in responsiveness, apart from the "public" stats above. Those take some time to display purely on the browser side, as they feed way too much data to a browser for easy rendering. (will be fixed at some future point). ;)
Probably the only down side to Redash is a need to understand SQL. That can start out pretty simply though. :)
The only thing I wanted to say is that the time spent learning a bit of SQL pays off massively. Perhaps something for your todo list :)