DECOUPLING_TEST_MAX_HOLD_VARIANCE is a region to be hold when end temperature is reached. This message . raised too slow - comes during inital heatup and means in
<font color="#555555" face="Open Sans, Arial, serif"><div style="font-size: 16px; line-height: 22.8571px;">// Time to see a temp. change when fully heating. Consider that beds at higher temp. need longer to rise and cold
// beds need some time to get the temp. to the sensor. Time is in milliseconds! Set 0 to disable
#define HEATED_BED_DECOUPLE_TEST_PERIOD 300000
the bed must gain 1°C whcih was defined here
// Minimum temp. rise we expect after the set duration of full heating is over.
// Always keep a good safety margin to get no false positives. If your period is e.g. 10 seconds
// because at startup you already need 7 seconds until heater starts to rise temp. for sensor
// then you have 3 seconds of increased heating to reach 1°C.
#define DECOUPLING_TEST_MIN_TEMP_RISE 1
So with default values this mean your bed heted up 0.38°C in 5 minutes.
</font></span></div>
