LTSpice is my go-to circuit simulator, but I have spent weeks of my waking life waiting on it to run simulations. My current demon is simulating a 1 MW DC power supply to validate current output smoothing solutions in low load scenarios. The topology is a delta-delta and delta-wye transformer, each feeding a summing bridge rectifier, the two rectifiers have their negative legs tied together and the positive legs are the outputs.

I only simulate one pulse (about 3 ms), but the simulation takes minutes to resolve the inrush. Tuning circuit impedances to match measurement is a real pain. At this point I'm just going to take many more direct measurements. If it was quick I would have written a script to scan through unknown parameters to maximize correlation with measurements, but that isn't reasonable when the simulation fails after several minutes of trying for most values.