Privilege separation is a fundamental security concept that has been used in designing many secure systems. A number of recent works propose re-designing web browsers with greater privilege separation for better security. In practice, however, privilege-separated designs require a fine balance between security benefits and other competing concerns, such as performance. In fact, performance overhead has been a main cause that prevents many privilege separation proposals from being adopted in real systems. In this paper, we develop a new measurement-driven methodology that quantifies security benefits and performance costs for a given privilege-separated browser design. Our measurements on a large corpus of web sites provide key insights on the security and performance implications of partitioning dimensions proposed in 9 recent browser designs. Our results also provide empirical guidelines to resolve several design decisions being debated in recent browser re-design efforts.