It's quite simple really.
The sensation of speed relies on relative distance travelled and perceived. If you drive through a straight highway in the middle of flat plains which has no landmarks, and there are no cars or buildings nearby, then often it becomes very hard to realize just how fast you are driving. The only hint you have, is the engine revving sounds, and the vibrations of the car, and perhaps the wind if your window is open.
The human eye uses a certain reference object around you, to measure the distance and time you've travelled, by comparing the position of that reference point when it was before to where it is now. So, if visible detail is poorer, then the sensation of speed also becomes more dull.
Remember what the trees were like in the older version. The tree barks were darker, and more defined, against the terrain. There were some problems with the 'floating' treeline being invisible, but at least, when you were low, you could clearly see the many tree barks around you and see how fast they swoosh by you. That creates a sensation of speed.
However, the current trees use a different method of depiction, and IMHO the colors of the trees aren't as pronounced against the color of the terrain tiles as it used to be.
In other words, the new trees look much better than the old trees when you are on the ground.. but when you see it from above, it looks very blobby, and it's hard to tell the tree/leaves from the grasses... where it starts.. where it ends.
The detail is more blurry, thus, the sensation of speed is dulled.
