Is heavy putter better than lighter putter on slow greens?

Technically it would help, ever so slightly, you just need to get a feel for the greens and know what pace you should play your putts.

This answer is correct. I just thought I would add that there are putters out there that aim to improve putting by generating a higher moment of inertia at Golf ball impact, and improving launch angle. They do this by providing a lower center of gravity and an optimized launch angle. These features can help lift the ball from it's depression, while stabilizing your putt and providing more accuracy on mis-hits.