The TrustZone technology, available in the vast majority of recent Arm processors, allows the execution of code inside a so-called secure world. It effectively provides hardware-isolated areas of the processor for sensitive data and code, i.e., a trusted execution environment (TEE). The Op-Tee framework provides a collection of toolchain, open-source libraries and secure kernel specifically geared to develop applications for TrustZone. This paper presents an in-depth performance- and energy-wise study of TrustZone using the Op-Tee framework, including secure storage and the cost of switching between secure and unsecure worlds, using emulated and hardware measurements.