Reactivity is one of the key features of hardware description languages. We present an efficient implementation of reactivity in the Scenic framework that allows the system designer to model hardware blocks. Scenic allows the designer to use C++ to model mixed hardware--software systems with a C++ compiler and a small library and without the need of a complex event-driven run-time kernel often found embedded in hardware description languages (HDL) such as VHDL and Verilog. Moreover, Scenic hardware descriptions can be easily mapped to HDL and synthesized into hardware implementations using commercially available tools. In this paper we present Scenic's implementation of concurrency (signals and processes) and reactivity (waiting and watching). When C++ is used as an HDL, context-switching overhead can become a significant performance issue during simulation. We introduce the notion of delayed expression objects, or lambdas, to reduce context-switching. Examples and experimental results ..