"Properly" ? Every vendor works properly. But 8000 symbols in streaming real time cost A LOT of money and A LOT of bandwidth (DSL is NOT enough). eSignal (or Interactive Data) offers enterprise-level solution for that many symbols but costs tons of money and as far as I know is not available to individuals. On the other hand you from what you wrote you don't need real time at all. You would need 5-minute snapshots. Something like that could be done with Tiingo, but I am not sure if they want their servers to be bombed with requests like this. Polygon is just a REST based web site that does not look to provide high-performance. And is not really supported by us.
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