Stemming takes about 15 seconds for a 3 minute song. And most DJ software downloads the entire song in a few seconds, then generates the display waveforms, and so on, and then you can play the song.
So "realtime" and "streaming" don't really apply, because the files download and process fast enough.
15 seconds for 3 minutes is quite slow, the open source thing i use, spleeter, says 100x speedup over realtime with a GPU, that would be 1.8 seconds for three minutes, which means that spleeter could, in fact, given all of the arguments of what i asked, do it "realtime".
It depends on the hardware. The laptop I use for DJ doesn't have a dedicated GPU.
I also use open source, preferring demucs over spleeter.
In my experience spleeter requires huge amounts of memory, my DJ laptop only has 16GB of RAM, and I need to close the browser to be able to successfully stem a 5 minutes song.
Demucs takes a bit longer, but it uses just 2GB of RAM. And the sound quality of the stems separation is much better with demucs, which is a more important factor than the runtime, IMO.
So "realtime" and "streaming" don't really apply, because the files download and process fast enough.