From the Sources
Keva vs. Kavanah: Obligation of the Heart
R. Elazar said: Always let a man test himself: if he can direct his heart, let him pray; if he cannot, let him not pray.
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
A Rabbinic Anthology
C. G. Montefiore and H. Loewe
New York: Schocken Books, 1974
Published by Jewish Lights