200bpm
Before you spec out an X99 system, make sure the DPC latency of your chosen board is sufficiently low.
Though this is important, I think people make a bigger deal out of it than it is, I've not encountered a modern system that didn't have sufficiently low DPC latency, barring something that was messed up.
That aside, the X99 boards are very low latency.
Anand reports the X99 Deluxe as being 74 microseconds,
HardOCP reports it as being 56 microseconds. Of course both are using the DPC Latency Checker tool which isn't the best for that kind of thing and also doesn't check on ISRs and other things that matter in terms realtime response.
Regardless, it isn't a problem on these boards.