[vintermann]

I think Digital Circuit Board on Steam is pretty nice. It's a pure sandbox, not a game, so you need an idea of what you want to build but if you do, it's nice and fast (circuits are, as I understand it, compiled).

It also comes with a memory and screen abstraction, which can increase performance further over hand-rolled memory and screens once you want that.

It doesn't have any encapsulation, though, which is odd since it compiles things anyway.