McDuglas

> I wonder what game is good to play co-op between a 40 and a 6 year old?

You could try factorio. It has trains, tanks, bugs to shoot.

0.18 was just Release Candidate for 1.0 - the only feature added on release is Spidertron, the rest is just fixes. I think you should update.

The devs were constantly optimizing the game. They've added a lot of performance tunings, options for video compression, etc. You should give it a shot, maybe download a factory built by someone else to evaulate performance.