[vardump]

> We use a National Instruments DAQ card, and need the PC to respond within 50 ms to issue new commands for hours or days. Remarkably, it usually (over hundreds of machines and decades of operation) does. When it doesn't, it's blamed on antivirus or firewall or technicians using the PC for other things while the software runs.

It works as long as full code path and data it requires is not paged out. Or some other thread doesn't consume I/O resources, etc.

In other words, it's not guaranteed at all.

Only way to get Windows to react reliably within 50 ms is in a kernel driver, as response to an IRQ. There's considerable jitter even in IRQ, but usually worst case service times are 200-500 microseconds. Depends a lot on other devices and on your IRQ priority. It's worse for passive level drivers (IRQL == 0).

50 ms guaranteed response time requires the code and data is in non-paged pool.