You're not listening to me.

The engine isn't taxing at all; "taxing" implies using up too much resources - I am not saying that. If anything, the Engine is under-taxing. It's not optimized properly for newer hardware. That's why the system specs for XCOM2 are so low; you can literally run it on a decade-old computer. It won't run well, but it will run.

XCOM2 is more taxing on the engine than XCOM EW. That's why EW may run smoothly for you but X2 won't, even though they use the same engine. EW wasn't demanding as much out of an unoptimized engine as X2 does, so you don't notice as much of the incompatibility between your newer hardware and the older engine.

Think of it like this: Pretend there's a mechanic. He owns a bunch of tools. He can fix classic cars with his tools because the tools were made around the same time as the cars he works on. He is excellent at fixing classic cars.

One day, someone drives a newer sports car into his shop, and asks him to fix some part of if that he doesn't have a tool for. He does what he can with his old tool set, but it's slow work because he has to fix this different car with the wrong tools. He can fix the car - he's an auto mechanic and fixing cars is what he does; but he's really bottlenecked because he doesn't have newer tools. He finishes work on the sports car - the job is done, but it could have been done better if he had one of those new tool sets.

Then one day, someone brings in the same kind of sports car, but it's brand new and WAY more advanced. The kind of advanced that usually requires more than one mechanic to work on it. Yet he still has his old tools, and even though it's the same type of sports car )which already took a lot of effort for him to fix), this one has even more pieces and new parts. He fixes it, but with his old tools it takes him a SUPER long time, especially by himself.

Mechanic fixing the car = Your PC running the game

Old set of tools = old game engine

Cars = games

Do you see what I'm saying now?