You haven't included enough information to get much help. What kind of data is being returned by the server? Remote Objects? HttpRequests? SOAP requests? From the little information in your post, I would be most inclined to believe it is a problem on the server providing the data, not the Flex application receiving it.
I'm fetching inventory and billing information from a SQL Server database through ColdFusion CFCs. I know the queries are running fast, because I've tested them in SQL Server Management Studio. The systems slows down when several users try to access information at once. It's as if Flex has to wait to display the information for one person before it can show the next recordset.
Flex is single threaded, am I correct? I'm wondering if there's a way to add threads to the application.
Flex is single threaded, but that only affects the client side, not the server side. Each flex client has no idea about any other flex client on other users machines. I don't use Coldfusion, so I don't know anything about how the CFC connections work, but it sounds to me like either the web server, or the process that is retrieving the query data and packaging it for delivery to the flex clients is the bottleneck.
Do more tests. Is it due to the number of records returned? Or the size of
Set up timers on the request, the result, and the display.
Use the profiler to gather more data.