The rosary should also be said with devotion; and here we may call
to mind what the Blessed Virgin said to Saint Eulalia,
that she was better pleased with five decades, said slowly
and devoutly, than with fifteen, said in a hurry, and with little devotion.
A devout young nun recited every day the complete Rosary of fifteen decades, but with little devotion, on account of its length. One day the Blessed Virgin appeared to her and told her to recite only the third part of it. “For,” said she, “a few prayers said fervently are more acceptable to my Son and to me, than many said negligently and without devotion.”
From the Secret of the Rosary:
116. It is not so much the length of a prayer as the fervour with which it is said which pleases God and touches his heart. A single Hail Mary said properly is worth more than a hundred and fifty said badly. Most Catholics say the Rosary, either the whole fifteen mysteries or five of them, or at least a few decades. Why is it then that so few of them give up their sins and make progress in virtue, if not because they are not saying them as they should.