Do i need more RAM when UnrealLightmass slows down?

I got like 12 GB Ram, when calculating light, UnrealLightmass uses aprox 8 GB, Unreal Engine 1 GB, when i work also in other software, sometimes things start to lag. Is this some kind of sign that i require more RAM? Or would UL always max out, causing probable lag to 3rd party software? Or would it be better to invest into a SSD drive, currently Windows runs on SSD but the UE software is installed on a standard drive.

Suggestions are welcome, thanks!