>> I suppose I am missing something, how come it takes much longer for you?

Not the OP, but at a guess (one of my relatives has kidney disease and take their blood pressure twice a day every day), it takes about 5' to relax, then another 5' to take two readings and combine the results. My relative takes three readings and keeps the last. From my understanding this is common with electronic devices, where the measured BP changes between readings.

Again from my relative's experience (I helped them with a bit of code to keep track of their BP and plot it etc) the trend is usually monotonic, i.e. either it starts low and goes up a bit, or it starts high and goes down a bit. Sometimes more than "a bit", like from 130/80 to 120/70 (i.e. from hypertensive to normotensive range), etc. Manual sphygmomanometers don't seem to have the same variance in their readings. From my understanding, the digital devices have some kind of model that estimates BP from the readings of an oscillometric sensor and are less accurate than the hand-operated ones. The table-top mercury ones are the most reliable of the bunch. And as to finger-cuff devices, you might as well read tea leaves.