If your circuit is unstable or you do something silly with the transient parameters, LT Spice will grind away for minutes. When this happens to me, I assume that I've screwed up somewhere. Usually the results are almost instant with my Intel(R) Core(TM) i5-1035G1 CPU @ 1.00GHz 1.20 GHz, laptop 8GB ram. I set the transient parameters something like .tran 0 {10/f} {1/f} {1e-4/f} where f is the input SINE(0 {A} {f}), .par f=12k A=.5 .options plotwinsize=0