Thermo-optic effects, especially the thermo-optic nonlinearity, are conventionally considered to be slow and detrimental in on-chip integration. Here, we challenge this conception by presenting a real-time observation of the ultrafast thermo-optic nonlinearity in time-varying effective epsilon-near-zero (ENZ) media. The results of this work might enable the design of integration-compatible near-terahertz ultrafast thermo-optic modulators with a low intensity demand.