Rainboe, your forthright contribution is appreciated as always, but I fear you may be labouring rather beside the point. Yes, the takeoff performance charts are very sophisticated and take into account all sorts of factors - but the charts only tell you what
should happen, not what actually
is happening as you hurtle (or dawdle) down that runway.
The problem occurs when the aircraft does not perform according to the charts. This may be because the wrong weight was entered, or because there is something physically wrong, e.g. tyres, brakes, other systems (as Curious Pax pointed out).
To avoid the high-speed reject or the EK-style lift-off-in-the-weeds, it would presumably be good to know about this underperformance as early as possible. Scumbag gives us his rule-of-thumb technique:
If I haven't reached 75% of 'takeoff speed' (get that from a book) by half way along the runway (get the length from a book) I stop.
That's fine - but the computer can make this kind of analysis much faster and more accurately than you can. And it can do it from the first few seconds of the takeoff roll - no need to wait until you're halfway down the runway.
And we are not talking about an "acceleration detector". No gadgetry is required. The computer already has all the data: it has all the sophisticated performance chart information, it knows the actual speed at each instant (you read your V speeds off the display, right Rainboe?), it knows to the millisecond how long you've been rolling. I'm not sure about the interface to navigation data, but I guess it also knows pretty much exactly how many feet of tarmac you've rolled over so far. So it can tell you, in a flash of simple arithmetic, such things as: your instantaneous acceleration, your average acceleration, the time remaining (at current performance) until you reach V1 and Vr, the distance remaining until you reach V1 and Vr, etc. Furthermore, in the same flash it can compare these
actual figures to the
expected figures in the performance chart, and alert you if they differ by more than a certain tolerance.
The computer
can do all this: it's only a matter of programming it to manipulate and display data it already has. So the question remains, why doesn't it? Cost? Rainboe Syndrome? Something else?