It is better to say a few chapters of Tehillim with kavanah (concentration and proper intent) than to say many without kavanah.
Sources and Explanations
The Tur (siman 1) writes: “One should cast his supplication before the Omnipresent; one who does much and one who does little are equal, provided that he directs his heart in his supplications, for a little with kavanah is better than much without kavanah.” This is learned from a Mishnah in Menachot (chapter 13, mishnah 11): It is stated regarding the animal burnt-offering, “a fire-offering, a pleasing aroma,” [and regarding the bird burnt-offering, “a fire-offering, a pleasing aroma”], and regarding the meal-offering, “a fire-offering, a pleasing aroma,” to teach that one who offers much and one who offers little are equal, provided that a person directs his mind to Heaven. And so it is ruled in the Shulchan Aruch (Orach Chaim 1:4).
It is clear that the guiding principle here is that quality is preferred over quantity. Likewise, it is clear that if a person is able to do much with kavanah, it is preferable to do so, for the preference is only of a little with kavanah over much without kavanah. And indeed, this is how the Magen Avraham (ibid., se’if katan 6) explained the words of the Gemara: only if the one who does little cannot do more, whether due to lack of time or due to a limited ability to concentrate for a long time, and he did all that was within his ability, is he equal to the one who does much, as we say: “One who does much and one who does little are equal, provided that he directs his heart to Heaven.” That is to say, even though the one who does much also directed his heart, nevertheless, if the one who does little did not have the ability to do more, he is as pleasing to Hashem, blessed be He, as the one who does much. This is also implied by the proof brought from the bird burnt-offering [which] the poor man brings, regarding which it is also stated “a pleasing aroma,” just as with the animal burnt-offering that the rich man brings.
The Taz (ibid., se’if katan 3) adds that sometimes, on the contrary, an abundance of supplications diminishes the quality of the kavanah. He writes: “Sometimes the abundance of prayer is the cause of one being unable to concentrate, and if so, it is better to shorten and concentrate. Therefore it says ‘one who does much and one who does little,’ for the reason that he should have kavanah, and therefore it concludes ‘provided that, etc.,’ meaning that the reduction that he makes is for the reason that he should direct his heart to Heaven, for it is better to do little with kavanah than one who does much without kavanah. And this is the reason that he reduces — so that he will have kavanah.” End quote.
Indeed, the Mishnah Berurah (se’if katan 12) combined the views of the Magen Avraham and the Taz and summarized the matter as follows: “But if the one who does much also concentrated, the one who does much is better. And if the one who does little has a constraint and cannot lengthen, or he assesses himself that if he lengthens he will not be able to concentrate, and so he reduces his supplications and says them with kavanah, it is considered before Hashem, blessed be He, like the one who has free time and lengthens his supplications with kavanah. And about this we learned: ‘One who does much and one who does little are equal, provided that he directs his heart to Heaven.’”
From the sefer “Shaf Veyativ – Laws of Saying Tehillim” by Rabbi Shay Tahan, Chapter 6: Intention (Kavanah) While Saying Tehillim, Halacha 7 (p. 90 in the print edition). Translated from the Hebrew original.
