Try to reduce the size of your model if you can by using some sort of symmetry plane or simplify your model by using solid via plate instead of a large number of vias. Default "Maximum Delta S" is 0.01 and normally I st it to 0.005 to get better accuracy. Also maximum delta Zo, has a role and I set it to 0.5%.
Also, if your air box is excessively large, you can reduce it without losing on accuracy. But, if you have done these already, there is not much that you can do.