Fact is Tefilla B’kavvana is min hatorah. Tefillah b’tzibbur is a d’rabannan. Many people find that davening in a fast minyan precludes them from davening with kavvanah. So they daven what they can b’tzibbur and then they have to leave and catch upo with kavvanah when they are able to.
Many people daven 3 times a day in a way that is 100% good d’rabannan but with no kavvanah. So they are mekayyem the d’rabbanann but leaving out the d’oraysa.
Apart from which I must mention that I am envious of the OP for whom observing and criticizing other yidden seem to be the biggeest issues he has. Worry less about what other people are doing concentrate on imoproving yourself. At the very least try and find a limmud z’chus forn those who must leave early.