Using network survey and objective productivity data collected from the Esquel Group, a large textile company headquartered in Hong Kong, we find that the density of worker social connections within a production line positively and quadratically affects the line-average productivity. However, we find that at individual level social connections do not affect the mean productivity. Instead, the density of social connections affects the variance of productivity at individual level, which reasonably explains the quadratic pattern of influence at the line level. This is because the line productivity depends on that of the bottleneck and variability of individual productivity causes occurrences of a bottleneck. Strengthening social connections may have little effect on line productivity at the beginning, but as the variance of individual productivity shrinks, social connections will be more and more effective in reducing bottlenecks. Hence, team building is important and a significant amount of effort should be invested before firms can see a significant impact.