As far as I can remember, Classic *does* correctly cull everything outside the viewport, even when zooming. I can't remember how its letterbox implementation works, it might just overdraw black bars (FWIW Construct 2 does a proper letterbox and only draws visible content)
If by "some systems" you mean integrated GPUs like Intel HD graphics, then you're probably running in to fillrate limits (i.e. the memory write bandwidth of the hardware), which is especially problematic at high resolutions. I honestly can't remember how Classic works in this regard any more, but if it has an equivalent to C2's 'Low quality fullscreen mode' where it renders the game small then just stretches the result up to the screen, then use that. C2 should soon be getting a front-to-back renderer mode as well which could significantly reduce fillrate and end up making C2's GPU performance better than Classic's! But there is very little hope of Classic getting that.