Of course tests are slow. If they're fast they're skipping something. You probably want to be skipping something most of the time, because most of the time your change shouldn't cause any side-effects in the system, but you definitely want a thorough test suite to run occasionally. Maybe before merge, maybe before release, maybe on some other schedule.
Each individual change might be fine in isolation but cause excessive memory pressure when accumulated with other changes, for example. Unit tests won't catch everything, integration & functional (hardware in-the-loop) tests are needed. I even sometimes have to run tests in a thermal chamber repeatedly to cover the whole -40-105°C temperature range, since the firmware I work on runs on hardware that allows that range & performance varies with temperature.
