It pretty much comes down to CPU and memory speed. The amount of RAM does make a difference to some extent, but speed is king. I've even noticed a substantial performance difference between running LT on a system with an integrated GPU vs. a dedicated GPU (literally the same PC with and without a dGPU), due to the two components having to share memory bandwidth.
Yes, it really does thrash that badly.
That said, I've unintentionally built save files where there's just no hope of the game performing well, even on a high-end gaming PC, due to *ahem* excessive breeding.