There's no way to to this right now other than coding something custom. We have discussed this in the past as a potential feature but it isn't anywhere the roadmap right now. It's also not clear what "slow" means as it's up to the application to decide. There'd probably be some kind of bandwidth/time/message based setting that would make this hypothetical event trigger.
If you want to hook in at a really low level, you could use owin middleware to replace the client's underlying stream with one that you owned so that you'd see all of the data going over the write (you'd have to do the same for websockets though and that might be non trivial).
Once you have that, you could write some time based logic that determined if the flush was taking too long and kill the client that way.
That's very fuzzy but it's basically a brain dump of how a feature like this could work.