When is the main time of saying Tehillim?
The recital of Tehillim is to be done specifically prior to Shacharis.
Does Tehillim of Shabbos Mevarchim push off Davening with a Minyan?
- On Shabbos Mevarchim, is one to finish the recital of Tehillim before Davening even if this will cause him to be unable to Daven with the Minyan?
The age old custom of saying the entire Tehillim prior to Davening does not take priority over Davening with a Minyan. This applies likewise to the Tehillim recited prior to Davening on Shabbos Mevarchim, that it certainly does not take priority to Davening with the Minyan. One is thus to be very careful that if he arrives to Shul while they are up to Davening, that he not make the secondary primary and primary secondary [and say Tehillim while they are Davening].
 Levush 1/9 in name of Shaareiy Orah; Elya Raba 1; P”M 1 M”Z 8; Beir Heiytiv 6/5; M”B 6/8; Beir Moshe 1
 The reason: As the main recital of Tehillim is to cut off the Kelipos prior to Davening in order so one’s prayer be readily accepted above. [ibid]
 Introduction to Tehillim Ohel Yosef Yitzchak [I have not found this section in the Shlah Hakadosh]
 Kovetz Zalman Shimon p. 30; Hiskashrus 942 footnote 1
 Introduction to Tehillim Ohel Yosef Yitzchak [I have not found this section in the Shlah Hakadosh]; See also Likkutei Yekarim 25 that the Maggid stated one is not to say Tehillim before Davening in a way that will come in the expense of Davening.; In Sefer Devek Tov this is brought in the name of the Baal Shem Tov.
The reason: As the main reason of Tehillim is to fight off the prosecuting angels, and this also is accomplished through Pesukei Dezimra. [ibid]