All sims are invariably CPU hogs. It doesn’t matter how powerful your CPU might be, it will get bogged down, even if it doesn’t appear to be stressed (I never see it going above 50% on any cores). I’m guessing some of the slowdown is due to textures. My 670s are only 2GB and TF is offloading a truckload of textures into the swap file. I’ve started using some rolling stock and repaint mods and this is obviously adding to the texture load. Having said that, this game does need performance optimisation. I’d go as far as saying it hasn’t been optimised much at all.