The tradeoff between accuracy and speed is considered fundamental to individual and collective decision-making. In this talk, I will focus on collective estimation as an example of collective decision-making. By decomposing the total accuracy into bias and variance, I will present that diversity and social interactions could promote the accuracy of collective estimation. The simulation results reveal how the exploration-vs-exploitation tradeoff relates to the speed-vs-accuracy tradeoff.The second part of my talk would be about the emergence of echo chambers as a result of homophily and limited connectivity in collectives. I also propose our heuristic solution that could help the collectives to cope with the issue.