Each organization runs their own Arcgis server with their contracted server hosting company.
General speaking, more money you spend on server hosting, larger capacity and faster speed you will get.
No.1 Amazon AWS. No.2 Microsoft Azure hosted very fast arcgis server.
ESRI use Amazon AWS to host their Arcgis.com and Arcgis Online.
Other small server hosting company such as rackspace, goDaddy, and your internet serverice provider (ISP), such as charter communication, etc can give you either “Excellent” , “Good” , “Fair” speed and load capacity with very economic rate, if you are a small city have less data.
You can test any organization's arcgis server speed by using time counter on progressing bar.
When you move the map or zoom around, you can read how many second needed to get data from Arcgis server for each operation.
The reading number on progress bar indicate this arcgis server's speed.
This example use Classified Color Blind system shows, US IRS defined opportunity zone in California Only
You can click other state, by find out oppoertunity zone in other state as well
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This example use colored symbology system, shows US IRS defined opportunity zone in Arizona Only
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more to try
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