When you daven slower than the minyan, or if you are late to the minyan, there are certain parts of the Tefila you can skip. There is a hierarchy of what you should say if you have time. If you will make up the missed portions after the minyan concludes, do you still need to go by the official hierarchy of what to skip (i.e. in the Hallelukas, etc.)?
And what is the bare minimum? Baruch Sheamor, Ashrei, and Yishtabach +?
If you must leave early from the only minyan in town, what is the earliest possible to leave? Taking the Tefilin off after Kedusha?