It is the tick time of your CPU, change it to a lower value (For instance 100us)
See:
https://www.qnx.com/developers/docs/7.1/#com.qnx.doc.neutrino.prog/topic/timing.html

If You requires an exact us delay, You cau use one of hardware timers of the CPU.
Bye

Hi,

if you need really shorts delay you could use a "busy loop" using ClockCycles() call.
Of course you'll need a thread and it is CPU consuming.
Otherwise, I agree with Mario. You should also consider the interrupt dispatch time as a minimal delay.

Bye

QNX 7 has high-resolution timers. You can use these, but depending on how many microseconds we are talking about setting
 up the timer, blocking and waiting for it to fire may not justify the cost.

--Elad