I have an original, Keurig (10-years old) with a large almost 2L tank(?) ... it's ON 24/7 unless I go away a few days ...
"Normally": I Lift & lower arm, select cup size = full amount of "instant' HOT water quicky dispenses; then reloads for the next time.
Now, AFTER recently descaling it –
"NOT Normal":  I Lift & Lower arm, select cup size = a few dribbles of HOT water from its internal reservoir come out, then reloads the necessary amount of water to HEAT up BEFORE dispensing the REMAINING amount of hot water to complete the fill ... AND the "Descale" reminder/notification (which I've never seen before) started flashing.  (behaving like the MINI = no standby reservoir)

So, NOW, when I want (instant) HOT water for anything, I am waiting upwards of 4 minutes before the selection  completes ... what's up with that?