What is it that really "eats hardware" when composing and rendering?

Im using heavy assets like the KITE assets. I dont develope games but using UE for simulating and visualization. They run just fine on my laptop with 4.13. I havent tested 4.13 on my desktop yet. Last time I used UE on my desktop must have been on 4.7. and then it crashed and hang alot. Does the lightmap depends on GPU or CPU? I know the UE editor demands on ram, so maybe adding more ram will improve the usability? Also, I want to avoid bottlenecks if Im going for a upgrade.