It's websockets, and yes we do make use of Durable Objects. But we do extensive batching and buffering on top of the raw platform. You can't literally send 120 messages / second to and from 100 concurrent clients. Actually the math tops out at about 8 :).
There are a variety of relevant limitations. Some of them are listed here: developers.cloudflare.com/durable-objects/platform/limits/
For this particular problem, it just comes down to CPU time. At the bottom of the stack ws.send() is a syscall and it takes significant time. You can only do so many of these calls per second. We measured it at about 8k/sec a year ago.
With a conservative limit of 2k calls to ws.send() (so that we can stay about about 50% utilization and only use half of the time for calling send), this implies 6 clients at 60 FPS using a naive approach (6 * 6 * 60 = ~2k).
It's not really a DO problem, it's just that doing n^2 messages can't really work in any platform.
