On 2017-03-31, I repeated test2 with the current version of parint and CentOS 7.3.1611 (Core) 3.10.0-514.2.2.el7.x86_64,
On a computer with a Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz

I connected pin 10(ACK) to pin 3 (data out). Then ran 2 copies of 
testpi. testpi #1 used to "get.time", and the other generated pulses ("pulse").

procedures

1. In xterm1, collect the time stamp data:
   script 0.005_gettime.txt 
   ../testpi
   get.time
   ....
   quit
   exit

2. In xterm2, generate pules and timestamps:
   script 0.005_pulse.txt 
   ../testpi
   pulse (rep=200000 usec=2500  (0.0025 sec x 200000))
   quit
   exit

3. combine data from 0.005_gettime.txt (col1) and .005_pulse.txt (col2) into 0.005.txt
   graph with gnuplot:
      set format y "%.6f"
      plot '0.005.txt' using ($1-$2) with lines
      set term png
      set output '0.005.png'
      replot

4. results:

   Graph of ISR_timestamp - pulse_timestamp shows worst case of ~340 usec, and a 3 at ~150
	All other samples are under 100 usec.
    


   Note: the period was 2500 usec, for test3 vs 5000 usec for test1,2.