Fusion 360 is a cloud based product and even it's partial able to run offline it works best with a reliable internet connection. If you want to utilize your RAM as a kind of RAM-disk for e.g. auto saves or cache this isn't something Fusion 360 can do (perhaps making the path configurable). It's a 3rd-party product you've to use. But this wouldn't solve your problem but enhances the risk of data loss.
Can you share one of your "slow" designs? Often it's just a matter of wrong techniques like big pattern in sketches, imported dxf files etc.
