The Air cannot be upgraded. The CPU and RAM are mounted directly on the logic board and it has no graphics card - it uses a graphics subsystem that is part of the CPU. A larger hard drive can be installed but that is the only hardware upgrade possible. If your Air has a standard hard drive upgrading to an SSD would offer some speed increase but I'm not sure that is a cost efficient solution. If your hard drive is more than 85-90% full, deleting some old/unused files could make things a bit better. Certainly returning to Snow Leopard would give your computer a big speed bump.