There is a speed limit because not all matter can make it to the speed of light, which is as I imagine the speed limit. And all matter cannot reach the speed of light because light is both a particle and a wave and these properties allow it to move at the incredible speeds it does. Also there is no heat limit because heat is basically the excitement of particles, but like sentinel said it does have some limits, 1K is the absolute minimum temperature a particle can ever reach although even in deep space particles cannot drop to less than 4 or 5 kelvin. On the other hand there is no heat limit because particle can be exited to vibrate until near light speed, and I believe that there has to be a heat limit because it heat is the speed at which particles move within a molecule, this speed that they move does have a limit so there has to be a heat limit, but this heat limit isnt known because scientists have no way of creating that kind of heat with the technology known to us.