I sincerely doubt that the lag would be that bad, but the modeling process could take over a month of hard work.
First of all, you would have to make a ton of different parts and joints, and second, you would have to make complicated animations.
The "easy way" would be to just have two models, one as a car, one as a robot, but then there would still be the issue of other animations; walking, running, jumping, probably not crouching, sitting possibly, etc. It would still probably take ages.