Model heat transfer in block but can't get the solution

I modelled the heat transfer within a block with some air gaps. In the model, heat is transferred by conduction in the solid domain, and in these air voids, convection and radiation are also involved. I have attached a picture of the block.

That problem is that the computational process is extremely slow. With very coarse mesh, it takes nearly 2 hours to get the simulation result.

I then refined the meshes for the air gaps to improve the accuracy of convection, and resolve the problem. After 4-hour computation, the solver sent this message;

Failed to find a solution.
The relative residual (0.28) is greater than the relative tolerance. Returned solution is not converged.

I don't think the heat transfer within a block will be a very complex model. It is astonishing to see COMSOL takes such a long time to solve it. My laptop is Macbook pro with Intel i7, 4GB RAM. Are there any problems in my model or is my laptop not powerful enough to run COMSOL?
Can anyone help me?