Rapid respaws have no effect on bosses.
The first rapid respaw used to work on bosses. After that CIPSOFT removed the effect on bosses to not devalue rares.
On March 22 2019 CM Mirade stated:
Rapid respawn only applies to monsters with a monster home. It does not apply to the raid and boss system.
CM clarification CM Clarification 2
Boss spawn times will remain the same. Bosses respawn rate was only affected by the first two Rapid Respawn Events. They decided for July 2018 rapid respawn to remove it.
As a result of player feedback and to alleviate the inflationary emergence of rare boss items drops, all bosses and minibosses will be excluded from this and future Rapid Respawn events.This includes bosses like Zugurosh, Jaul or the new falcon knight bosses.
Edit: Clarified a question.
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