This is my old Medieval Fenlands map in East Anglia with 10 layers at 2000 polygons each and two web mapping services - zoom in an out to appreciate its speed.
@giscloud is still new so there is a way to go yet - as some of the tweets that increased 1500% in the last few days indicate - labelling, scale dependency, layers and legends, etc. Note also that as this relies on HTML5: pre-rel. 9 Internet Explorer that don't support it will automatically revert maps to their original rendition.
Fifteen years ago Argus' distinction from ArcView and Mapinfo was to render in less than 5 s. all 50 US states and 3000+ county polygons. And about that time Hipparchus claimed to map the solar system at metric scale... One could say that:
What goes around, goes around, goes around again,
da-na-na da-na-na da-na-na my friend say
Where you gonna run to? When it gonna end?