The term "serving" is a bit misleading here. Most of the time, vector tile servers are serving pre generated tiles, which is extremely fast. This analysis is about generating tiles on the fly from PostGIS through a custom web server.
I've built a server for generating OpenStreetMap vector tiles on demand from a GeoDesk database, which is barely larger than an .osm.pbf (100GB vs. 80GB for current planet.osm.pbf) - much smaller than a PostGIS instance: <a href="https://github.com/styluslabs/geodesk-tiles">https://github.com/styluslabs/geodesk-tiles</a>
another option that would have been interesting to see here is serving PostGIS GeoJSON export -> tippecanoe encode. Tippecanoe is super fast, parallelizes well and built solely for generating vector tile data (with lots of configurable options that PostGIS lacks)
okay, do they mean vectors, or tiles, because that's like saying "serving PNG JPEGs" or "serving JPEG PNGs". Some servers chuck back /a picture/, some servers chuck back /an SVG/ or line data.