Полсекунды - это, таки, наилучший случай. Высокая добротность *
низкая частота = длительный запуск. Вот, например, выдержка из STM32L215:
tSU(LSE)(2) startup time (Vdd is stabilized) 2 s nom.
2. tSU(LSE) is the startup time measured from the moment it is enabled (by software) to a stabilized 32.768 kHz oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly with the crystal manufacturer.