HFSS mesh reduction
1. Model the traces as 2D impedance/finite conductivity surfaces rather than 3D metal.
2. Delete any trace you think won't influence things, or combine parallel traces into a single trace. Obviously, for this you have to know enough EM theory to know which simplifications you can make, and which introduce too much error.
3. Simulate only a small part of the board at one time.
4. Use HFSS-IE (I've never tried this, but generally method-of-moments solvers are more efficient for this kind of problem.)