The Baal HaTanya (Igeres Hakodesh ch.24) elaborates on the great merit of davening with kavanah, and he writes that when one isn't able to daven with kavanah, he should at least appear like he is davening with kavanah. It will be a chilul Hashem to publicly show that he doesn't respect the tefillah.
We quote his holy words:
"My friends, my brothers, don’t do bad, o friends, beloved to their Creator and hated by the yetzer hara. [Chazal say] אדם יעשה אל המקום לפני אחת שעה רשע עצמו, don’t be a rasha, not even for a אחת שעה one moment before Hashem. אחת שעה alludes to the hour when we gather in beis medresh to pray before Hashem. That is the אחת שעה, the time that Hashem desires and chooses more than the rest of the day. It is an eis ratzon, an auspicious time before Hashem. At this time, Hashem reveals His presence in the beis medresh, the miniature Beis HaMikdash, and we can meet with the Shechinah. [His presence] is enclothed in the words of tefillah of the siddur, and His Shechinah becomes revealed to each individual according to his intelligence and shoresh neshamah [when he davens with kavanah]... Those who search and yearn for Hashem will find Him. But if at this time you will speak [to your fellow man about] your own personal needs, you are expressing that you aren't interested in seeing Hashem's presence that is being revealed at this time. You are showing that you don’t want to think about the beauty and greatness of the King of kings, Hakadosh Baruch Hu, Who is revealed above at this time, and also He is revealed below for those who desire to look at His honor and greatness.
"The kingship of heaven resembles the kingships of kings in this world (כעין דרקיע מלכותא דארעא מלכותא). It is the way for kings to be concealed in their innermost room, with many guards at the doors. People wait their turn for days and for years to be granted the opportunity to speak to the king. They wait for their turn to behold the beauty of his glorious kingship. However, there are times when the king comes out to the people, and everyone can see his glory. An announcement is made throughout his kingdom, telling people they can come, gather, and stand before the king to behold the honor of his kingship and the glory of his greatness. Imagine how disgusting and foolish it is if someone stands before the king, and instead of looking at him, he chooses to look elsewhere and is occupied with his own needs. How disgusting and foolish he is! Everyone will look down at him. Additionally, he disgraces the king because he expresses that the king isn't important to him, and he doesn’t care to look at Hashem's honor and glory. He is more interested in his own needs. For this, he deserves the death penalty because he is showing disrespect to the king in front of everyone. Therefore, it states (Mishlei 3:35) קלון מרים כסילים, 'the fools show disgrace.' The pasuk is telling us that even if someone is a כסיל, fool, he shouldn’t קלון מרים, show this disgrace to everyone. Chazal say that when one davens, it should be עומד כאילו המלך לפני, as if he is standing before the king. This means he should at least show, in his deeds, כאילו, as if he is standing before the King. This is how it should appear to others who look at him and see his deeds and words... The purpose of tefillah, the reason they were established, was so that one can think well about the words. Someone who doesn’t even pretend that he is davening with kavanah, he is chayav misah. The Zohar writes about him, that he is acting disgracefully...and that he doesn’t have a portion with the G-d of Yisrael r'l. Therefore, as a shaliach of Chazal, I make a gezeirah that no one should speak בטלים דברים from the moment the chazan begins the tefillah until after davening. This is for Shacharis, Minchah, and Maariv. Someone who transgresses this, on purpose, should sit on the ground and ask three people to be מתיר נידוי, to free him from the heavenly ban that he was placed into, and this will cure him..."
