Recent research at the cross between cognitive and social sci- ences is investigating the cognitive mechanisms behind coop- erative decisions. One debated question is whether cooperative decisions are made faster than non-cooperative ones. Yet em- pirical evidence is still mixed. In this paper we explore the implications of individual heterogeneity in social value orien- tation for the effect of response time on cooperation. We con- duct a meta-analysis of available experimental studies (n=8; treatments=16; 5,232 subjects). We report two main results: (i) the relation between response time and cooperation is mod- erated by social value orientation, such that it is positive for individualist subjects and negative for prosocial subjects; (ii) the relation between response time and cooperation is partly mediated by extremity of choice. These results suggest that highly prosocial subjects are fast to cooperate, highly individ- ualist subjects are fast to defect, and subjects with weaker pref- erences make slower and less extreme decisions. We explain these results in terms of decision-conflict theory.