Hard question, there's a lot; one favourite I came across years ago on one uni. course when I was finishing my SW Eng. MSc and while had I worked in Global SW Dev for years. The research was so well put together on a difficult to measure topic:
An empirical study of speed and communication in globally
distributed software development
An empirical study of speed and communication in globally distributed software development
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.324...