Dmitry,
PPS on Pi has significant lag, variable lag, isn't a RT OS, is multi-core, jittery CPU, and then you're taking it as a reference delivered through variable Ethernet speeds as its under a USB controller. You may find it interesting to turn off NTP on your boxes, turn off any running load on your boxes, then bring the Pi reference time across and compare it to system time.
Then compare again with your trading load running.
And if you're comparing to Windows system time, you're going to be amazed at the variability and ugly jumps that makes. Windows system time lags variably and horribly depending on load.
