Compact silicon-on-insulator (SOI) waveguide thermo-optically tunable Fabry-Perot microcavities with silicon/air Bragg mirrors are demonstrated. Quality factors of Q=4,584 are measured with finesse F=82. Tuning is achieved by flowing current directly through the silicon cavity resulting in efficient thermo-optic tuning over 2 nm for less than 50 mW applied electrical power. The high-Q cavities enable fast switching (1.9 μs rise time) at low drive power (<10 mW). By overdriving the device, rise times of 640 ns are obtained. Various device improvements are discussed.