When I flew Valiants we had a check time to 100 knots to make sure accel. was OK. If speed was down we aborted, did once, we went off the end, slowly. That check might well have saved Emirates at MEL.
This topic came up in the Madrid Barajas t/o accident too, when it was intially stated an engine failure occurred... rule of thumb checks or stopwatch checks to a nominal 80 or 100kts have been proposed after many 'fail to accelerate properly' accidents/incidents