Yeah, with a PTZ, it really is critical that the MP/sensor be on the ideal MP/sensor combination due to the wide range of lighting conditions it could experience at night.
Looking at the video though, I do not think the 2MP would have started tracking much sooner though.
The camera needs time to identify the object and determine if it meets the "signature shape" of the object to be tracked.
At this point, it is more headlight glare and vehicle shape cannot be determined.
It isn't until the headlight bloom starts to disappear and the shape of the vehicle can be seen that it starts to track.
You could try adjusting the parameters some more to try to knock out the headlight bloom (recognizing though that it might come at the cost of being too dark and then miss a person) or live with this tracking.
You could try adding some HLC and see if that could knock the headlight bloom out.
You could add that external IR on the house facing this area so that the camera could see the outline faster.
You could add white light and run it in color and it may recognize a vehicle sooner.
But I don't think the tracking was bad considering the night time and it did track once it recognized it.