We present plasmonic thermo-optic (TO) switches made of compact Cu-dielectric-Si hybrid plasmonic waveguide ring resonators (WRRs), which offer high tuning efficiency and fast switching speed by placing a metal heater closely above the Cu cap and choosing a material with high TO coefficient as the sandwiched dielectric. Prototype devices with 2.5- $\mu $ m radius and Si3N4 as the dielectric are fabricated on the silicon-on-insulator platform using standard complementary metal-oxide–semiconductor technology and are measured at 1.55- $\mu $ m telecom wavelengths. They exhibit a $Q$ -value of $\sim 166$ , a tuning efficiency of $\sim 1.1$ nm/mW, a response time of $\sim 6.5 \mu $ s, and an output modulation of $\sim 5$ dB under a 5 V/10-kHZ driving voltage. Methods for further performance improvement are addressed.