1. Was it windy? If there was a wind going into the hill it might push it into the hill a bit when going up (dangerous!) and have trouble getting down into the wind on the way down. If the wind was oriented like this you might want to map across the hill instead of up and down it. We actually recommend that all the time. It goes faster.
It is hard to tell from the images but it looks like there was still terrain changes happening. It is possible that was linear section of the flight plan if the terrain was linearly rising it may have just been between 2 waypoints. Remember that we simplify the contour to plus or minus 5% of the flight height to ration waypoints.
If it wasn't windy I don't really know... We give the aircraft the waypoints and it flies it. It might be more sluggish doing elevation change on the way down than the way up. The I2 is a pretty fast aircraft so I wouldn't expect this. We haven't seen this issue before.
2. Remember that the speed that is reported is the ground speed, not the airspeed. It might be going forward at 1 m/s but also up at 3 or 4 m/s.