Theme and plugins matter, but weak TTFB, CPU limits and a slow database can block every optimization effort.
For simpler sites, Hostilla gives a convenient foundation; with higher traffic, VPS or DataHouse infrastructure becomes useful.
After changes, measure not only a test score but real server responses, error logs and behavior under traffic.
For everyday hosting, start with Hostilla. For dedicated infrastructure, servers or colocation, continue with DataHouse.