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In my experience, 5000 anything other than static objects is a lot. Neither the node system nor the animations are optimized for such numbers. There have been talks about implementing something to cover this use case but nothing concrete as of yet (proposal).
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InfluxDB
InfluxDB high-performance time series database. Collect, organize, and act on massive volumes of high-resolution data to power real-time intelligent systems.
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Spring RTS game engine
A powerful free cross-platform RTS game engine. - Report issues at https://springrts.com/mantis/ (by spring)
That said, having that many units will be very challenging in pretty much any general purpose engine and you may want to consider an engine built specifically for RTS games. A quick search found the spring RTS engine: https://springrts.com/
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