There are a few reasons that AI may slow down like that. You can most likely fix this with an AIW editor.
The problem is getting your understanding to a point where you can diagnose and fix the problem. It would be easier if you could tell me the track and what AIW editor you use. It looks like the guitarman editor, so I will presume it is for now.
The most straightforward problem to fix that can cause this is a slight kink in the AI path, You may not even see it, and it may be so small that you cannot see the AI turning when you watch them. You can try zooming in and look very closely at the line for a kink or minor misalignment in the AI path. Or you can replot the path and test to see if it's gone.
The second most manageable problem to fix is there could be one waypoint that has a slower speed. You can find this by clicking on a waypoint close to the problem and using the X and C buttons to scroll through the waypoints in the area. In the left panel, you will see a heading WP_Path with some value next to it. Watch this value and look for one waypoint with a lower number. If you find it, you can manually enter a new value approximately halfway between surrounding waypoints or replot the path in this area and check if it has been fixed.
Next is the track boundary and no-cut boundary. If the AI gets too close to these, they will slow down, again using the AIW editor to replot the boundary. In your example, it does have a weird bulge in the no-cut boundary, but I think it is okay. However, this is another common problem to watch out for and learn to fix.
Now far more complex. It can be that the Z coordinate of the waypoint is incorrect. You can see the coordinates in the panel on the left. They are the values for WP_Pos, again scrolling through waypoints and looking for one that has a radical difference. This brings me to the problem with Guitarman. It may be that when editing waypoint values such as WP_path and WP_Pos, Guitar man will not save the new values into the AIW when you save.
To get around this problem, Open the AIW in notepad, and use the find function to locate the waypoint you want to edit. I usually use the coordinates from WP_Pos and search for them. Once you find it, you can manually change numbers directly into the AIW file.
