? on QNX 4.25 Interrupt Service Inhibiting or Delays

We are working with a system that performs time capture with a 1 microsecond
resolution when an interrupt is serviced.
We inject a test interrupt trigger on a periodic basis with an accuracy of
+/- 15 microseconds.
Captured time data shows varying interrupt service latencies from of 5 to
220 microseconds.
The interrupt service process is set at the highest priority (29).
The interrupt service process captures the time ( 8 inport calls) and exits.
PROC_32 is set to (26)
Our processor is a Pentium 166.
We are setting up some test cases to isolate this part of our application
but are interested in any experience that anyone else may have had with
interrupt blocking or latencies.

Is anyone aware of any reason or observation that QNX would inhibit or delay
interrupt servicing?

Bob Blair