Lubrication. I used silicone spray lube on the insides of the holes in the top case half and the key action improved tremendously. You don’t need much - wipe it off well before reassembly and the thin film left over is all that’s needed.
I had thought about that, but I was worried about using anything that might be particularly chemical. I’ll have to give that a go. Given my other option was getting a mechboard for it, I think the lubricant spray is probably going to be cheaper.