If you open the animation palette on the far right side is an arrow.  Click that and the bottom option is to display the Key count in the little boxes.  So the code to do the counting is still there just that it is not displayed all the time.  So most likely Poser still does a count each time there is a change in keyframes.

As far as the slow down, the more keys you put in the more complex the math gets to figure out what the path is going to be.

above is a channel and a Key frame every 10th frame, all at the Zero point.  Also I put in one point at 75 which is about the half-way point on the time line and pulled it down.  Notice that the curve not only effects the Keys next to it, but also the 2nd set out.  Although it appears to become stable 6 points from the center that is not the case.  Each Key is Zero but the frames between those keys have some slight value change from Zero.  Even Frame 5 which has 7 keys between itself (5) and the center key (75) it still has a change you can see on the dial.  Granted it is small but it still is a change.  It appears that Poser will use all keys in a channel to try and create the Tween Frames so if your animation is 1000 frames long and you have 100 keys much more complex than the simple example above you can imagine the math involved in making a line try to flow through all those points.  And those Tween Frames are computed every time there is a change to a key Frame.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13