Arcgis Hub (pre-loaded, load fast, search keyword is lighting-fast)
Improved searching by elastic search. Make searching lighting fast (average speed is 0.002 second per search compare to previous for-loop search average 0.4 second).
Arcgis Hub (real time live sync, slow)
ESRI arcgis portal limit each request return 10 sites, so you have 10, 20, 30........ until reach 4000 sites. That is why slow.
Initial loading 4000+ sites need 3 second, after that search keyword is lighting fast (elastic search).
1) Anyone can registar their own org's arcgis portal on esri hub.arcgis.com , opendata.arcgis.com and host there.
If they do so, it will show up real time live here: Arcgis portal hosted by ESRI (real time live sync slow)
Some portals are NOT working, some portals are full of gargage.
ESRI does not regulate and or clean those user's content.
Do not surprise if you run into those garbage portal sites.
2). Most of city county state choose to host arcgis portal on esri hub.arcgis.com , opendata.arcgis.com
Some city county state choose NOT to do so.
They setup arcgis port on their own and or rental servers.
Those city county state will NOT show up here.
You can find them on the other list Esri open data 1000+ organization