I have the active input system set to “Both” and the new input system is set to use fixed update.
If I understand things correctly, if anything the old system should be a frame behind the new system, since the old system populates the inputs on update, and the new one does it on fixed update.
Instead I’m seeing the opposite, where when I press a key, the new input system doesn’t recognize it has been pressed until one fixedupdate later.
My understanding is that this is likely by design due to the way that there is a separate thread that checks for input changes and processes those inputs. Those inputs will always register on the next scripting Update() rather than the original input system that checked as part of each scripting update. Which as you probably know caused other problems such as missed input events due to low framerate.
If you haven’t yet, I’d recommend filing a bug report and putting the number here if you can, and let us know the results of the bug.