How much bandwidth does StarCraft 2 use?

To add to this, sc2 processes commands in a tick about once every 40 to 50 milliseconds (~20-25x per second at fastest game speed). Since there's some latency involved and it would be weird to be dancing between almost zero latency and 50ms (depending randomly if you clicked right before the tick or right after) i think they buffer commands by 1 tick or so, so there's a lag of about 100ms (0.1 seconds) before internet latency is taken into account.

Since FPS games run differently with different methods to sync every game client*, having a 30ms ping can be twice as responsive as a 60ms ping. Having a ping of 30-50 instead of 100 can make it feel like a very different game, where you get a lot less "but i totally shot him first!" moments when you hit stuff on your screen but his shot in your face reached the server first.

With the same difference in an rts though, 50 ping instead of 100 - since your commands are being delayed by 100ms anyway, it'd be a 150ms delay vs a 200ms delay - only 1.33x faster instead of twice as fast - and nowhere near lagless. It does vary by engine, but a lot of RTS's use that model i assume.

Heroes of the Storm also uses it - which is disappointing - it naturally can't be as responsive as League of Legends for example due to that. There are some other negative effects - while it uses less bandwidth, it has higher CPU load - massively higher in some cases - and since a lot of simulation has to be done on ticks, there are slight spikes in latency on your system (not internet latency) and loss of visual smoothness whenever those ticks happen, because CPU load spikes momentarily and one frame gets delayed. You need a system more powerful than you would expect to mitigate those spikes - you might need as much as 120-180fps for it to be as good as a constant 60fps on a 60hz screen, and being at 80fps for example would show visual stutters over 20 times per second coinciding with hardware lag spikes. Not only that, those high framerates are simply not sustainable - much of the processing is done on one thread and it becomes a very high load when lots of units are involved, even on heroes of the storm there are issues there on the best processors, so you can't simply overpower the problem and get your responsiveness and smoothness back.

Overall i feel that blizzard took a compromise here that was bad for playability - they didn't have a MOBA engine, they took the sc2 engine and repurposed it instead of building one - and as a direct result, both performance and responsiveness was compromised on both low and ultra high end systems - even if many of the positive features are intact. This style of engine is good for an RTS with two players and hundreds of units - it has some serious tradeoffs though many of them might even be considered necessary - but it's just not suited for a MOBA.

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*FPS games with low player counts can just have a server send packets to everyone saying "Player 3 is at XYZ coordinates" in a continuous stream AFAIK, many many times per second

SC2 doesn't do that - it sends the clients a list of commands like in a replay file. Something like "Player 2 told marine X to attack move to XYZ coordinates" and then it expects everybody who's playing in the game to have their computer simulate it, work out a path, see if any unit moves into attack range of the unit etc. There's not even any communication sent saying that it did encounter another unit, or even that it killed another unit - they're just simulated simultaneously and identically from the same command given earlier.

It's pretty surprising how much you can assume happened for every player in the game as long as you have a matching game state, a command and a completely deterministic engine (the same command will do the same thing, even on another computer 5000 miles away)