Increasing computing resources is always the best solution to have precise simulations with HFSS.
But if you need rough estimates and results you can go with smaller mesh.
Mesh definition is always very crucial to get correct results and no to burden your computational sources.
you can start with coarse mesh and check the results and then can go for finer mesh if needed and then take advantage of symmetry planes.
If doing all this does not solve your issue then there is no else option but to go for more RAM