Computer Simulation Technology AG (CST AG) develops simulation tools for electromagnetic applications. The CST STUDIO SUITE(R) software covers a wide range of applications, ranging from antenna design e.g. of today’s mobile phones, to the simulation of particle accelerator components. Not the convergence order of the algorithms is important, but rather reaching the required accuracy as fast as possible. Besides choosing the optimal algorithm, it is of course essential to have an optimal implementation. During the last couple of years, CST’s software was highly optimized, parallelized, and multi-GPU support was added. The relevant parts of the code are now often 100 times faster than a straightforward serial implementation. Maintaining this good performance is as crucial as functional testing. In a large team of developers, like the team at CST, it’s hardly possible that every developer is always aware of all performance implications when adding new features. Therefore, automated performance monitoring of the software is essential. In my talk, I will share some experiences of the last years of tuning and optimization at CST.