Decryption Time

What’s the recommendation for decryption time, and what should I be concerned about it when choosing X Speed vs Y Speed. And Is there any danger for higher speeds depending of my machine hardware?
Database Format

Should I be concerned about about database formats when using a password manager, and why?
Key Derivation Function

What is exactly the key derivation function, and what it does?
Transform rounds, memory usage and Parallellism

What they does? (except by memory usage), and why is it matter to have a choice to choose how many memory usage in this case, what’s the difference?
Security Key File

Which one is the best, why and what should I be concerned about it.
I formated the topic to make it easily to someone who have the same doubts to learn about it either. I appreciate everyone’s time whose answer the topic.