Lee Kun explained and compared it nicely in the following question : https://www.tibiaqa.com/3229/how-does-the-improved-respawn-system-works
So basically respawn rates are as follows:
Base rate: 1x
Improved area: 2x
Rapid Respawn: 5x
Rapid Respawn + Improved area: 7x
Boosted creature@ 2x (please see CM's confirmation below)
https://www.tibia.com/forum/?action=thread&postid=38496131#post38496131
> Can you provide us with any stats concerning the
> increased value of the daily monster? For the
> experience, will it be +% or x1.1/1.2/..., and how
> will this work out with prey on that respective
> creature?
Double XP, double loot and a two times faster respawn rate are applied to the boosted creature.
Additional bonuses such as prey or other events like rapid respawn, double XP/skill weekends will be applied on top of that.
Important note: "Note that additional bonuses such as prey or other events such as Rapid Respawn, Improved Respawn Rate and Double Experience and Skill Events will be applied on top of that. That being said, during these special events creatures categorized as Hard or Challenging in the Bestiary cannot be boosted as confirmed by CMs"
https://tibia.fandom.com/wiki/Boosted_Creature