(ד) רבי אליעזר אומר העושה תפלתו קבע אין תפלתו תחנונים. רבי יהושע אומר המהלך במקום סכנה מתפלל תפלה קצרה. אומר: הושע השם את עמך את שארית ישראל, בכל פרשת העבור יהיו צרכיהם לפניך. ברוך אתה ה' שומע תפלה.
(4) Rabbi Eliezer says: One who makes his prayer "set" [as though it is burdensome to him], his prayer does not constitute "pleading" [for Divine mercy]. Rabbi Yehoshua says: One who is traveling in a dangerous place should offer a brief prayer [and] say: Save, G-d, Your people, the remnant of Israel; at every period of transition let their needs be before You. You are the Source of all blessing, G-d, Who heeds prayer.
(ד) העושה תפלתו קבע שתפלתו דומה עליו כמשאוי. ולשון קבע שאומר חוק קבוע עלי להתפלל וצריך אני לצאת ממנה:
(4) The one who makes his prayer a burden: That his prayer is similar to a load. And the language of "burden": that he says, "a law is established upon me to pray and I need to fulfill myself from it."
Talmud Bavli, Berachot 30b
If a man is riding on an ass [and the time for prayer comes], if there is anyone who can hold his ass, let him get off and pray; but if not, let him remain on the ass and pray. Rabbi said: In either case let him remain on the ass and pray; the only important thing is that his heart should be directed.
Tosefta Berachot III, 18
In the great debate over praying according to one's obligation (keva) trumps praying according to one's ability to concentrate, focus and emote (kavanah), here we see those who favor kavanah. Note that neither Rabbi nor R. Elazar advocate disregarding one’s prescribed, halachic duty to pray the liturgy at certain times. Rather, to keep that obligation, one cannot parrot the words. The message seems clear to me. God wants the heart.
Rabbi Mike Comins
"Prayer without kavvanah is no prayer at all. He who has prayed without kavvanah ought to pray once more. He whose thoughts are wandering or occupied with other things need not pray until he has recovered his mental composure. Hence, on returning from a journey, or if one is weary or distressed, it is forbidden to pray until his mind is composed. The sages said that upon returning from a journey, one should wait three days until he is rested and his mind is calm, then he prays."
"How grateful I am to God that there is a duty to worship, a law to remind my distraught mind that it is time to think of God, time to disregard my ego for at least a moment! It is such happiness to belong to an order of the divine will. I am not always in a mood to pray. I do not always have the vision and the strength to say a word in the presence of God. But when I am weak, it is the law that gives me strength; when my vision is dim, it is duty that gives me insight"
(Man's Quest for God: Studies in Prayer and Symbolism, New York: Charles Scribner's Sons, 1954, pp. 64-68).


