Development of Swift is underway and will initially be a more simplified version of the earlier idea. Instead of rewriting the rendering engine entirely, it will instead add adaptive graphics support for per-device/device-specific optimisations.
What adaptive graphics does is automatically adjust your graphics settings depending on your current average framerate to provide the best possible visuals without any lag. This also can provide better performance than if someone manually adjusted their video settings - as not all scenes will provide the same complexity - some areas might be a busy built-up town with lots of stuff to render, other areas might be a nearly empty world like a spleef minigame for example. With Swift, when you join that spleef minigame and get insanely high framerates beyond what your monitor can provide, Swift automatically increases your settings so the game looks better while still being as smooth as your monitor permits. When you enter that busy town, Swift’ll detect that you’re lagging and will automatically turn settings down in a conservative manner, in an attempt to improve performance without downgrading the visuals too noticeably.
Additionally, Swift will notify you if it detects that you’re still lagging after it tries everything it can to improve performance, giving you advice on what other things you could do to improve performance that are out of Swift’s reach (e.g. it might recommend you to get a better graphics card, or switch from integrated to dedicated graphics, or it might recommend that you close anything else you have open that’s CPU-intensive like Google Chrome, or to update your drivers and how to do so, etc…)