In one of the Shuls that I Daven they often say Tachnun so quickly (especially Monday & Thursday) that by the time they say Kaddish I am still way behind. If one finds himself in such a situation is there a preferred list of what to say and what to skip? Or should one just fall behind?
See this - You're not the 1st person to have asked this. Excerpt:
Is it more important to say Vehu Rachum or to say Tachanun?
What happens if there is insufficient time to recite both Vehu Rachum and the rest of the Tachanun together with the tzibur?
It seems that one should recite Tachanun with the tzibur and “Vehu Rachum” after davening.
It should be noted that the commentaries dispute what is included in the takanah of reciting Vehu Rachum. Some contend that the takanah is to say Vehu Rachum, and to say it while standing (Shulchan Aruch, Orach Chayim 134:1), whereas others explain that the takanah included only reciting Vehu Rachum, but did not require one to stand (Levush). (They all agree, however, that one should recite Vehu Rachum while standing.)
Vehu Rachum should be treated with the kedusha of the Shemoneh Esrei (Magen Avraham). Therefore, there are those who contend that it should be said quietly (Rama 134:1). However, the Beis Yosef rules that one may say Vehu Rachum aloud, as is the custom of many people.