Consider using Google Benchmark for performance tests

[joergbrech]

Currently our unit test suite contains a few performance tests using ctime. By seperating benchmarks from unit tests

• we speed up the test suite, because benchmarks are a bit expensive;
• we make it easier to add new benchmarks;
• it becomes easier to track performance changes, because we do not have to browse through the console output of our test suite;
• we use something a bit more modern and accurate than ctime and
• we improve the overall structure of our CI process: Afterall, a benchmark is not a unit test.

--> Google Benchmark

Also, this looks like fun: https://github.com/rhysd/github-action-benchmark (but is probably overkill...)