by analysing the code you can calculate the distance spent cutting and the distance spent in rapid movements. Fusion360 does not know how fast the machine rapids (though there is probably a setting for it somewhere), so must make a guess and do (speed * cut distance) + (rapidspd * rapid dist). BUT we have not yet considered acceleration. That adds time to almost every move, especially sharp corners, but does not add to long series of line segments that are nearly inline with each other, can the controller keep the speed up through those? Fusion has no idea, so, guess some more and make an estimate. OTOH CONTROL can know some of those values, rapid speeds and acceleration etc. Even some of the angles that will require deceleration or not. But doing all of that for a time estimate can take some time, esp for a large carve. I don't want to wait for that, do you? (-: So, keep the calculations simple and instead let the carve begin, much better to be actually cutting. If you are concerned about how long a carve will take you can split the gcode on line count (options in the post options dialog) to get shorter chunks. Over time you can make a note of the estimates and the actual cut times and learn which one to trust, or how to adjust for what your machine does in real life.
Fusion estimates, Control estimates. Realworld will be somewhere between the two. We add around 30% slowdown for acceleration (good average. But usually a bit high, better if a job Finishes sooner than you think)
