Pixhawk PPM Sum input lag

Pixhawk PPM Sum input have a lag (delay). Delay is between the rc roll/pitch stick is moved and copter begin to roll/pitch. Thare is no input lag at APM2.5 and ther is no input lag with Pixhawk S.Bus input.
Lag was found at AC3.1.5, AC3.2rc2 and AC3.2rc3, with 3dr PPM Encoder and Frsky PPM output.
Pixhawk Lag is abnormal because APM also have internal PPM encoder without this lag.
Lag is about 50-70 msec as I see. It is not the PPM frame time lag (20msec), because APM works fine with Frsky PPM without any visible lag.

Did anybody notice this lag at Pixhawk?
How can I fix it?

Noticed the same thing. There is an issue about this on github.com/diydrones/ardupilot/issues/1238