Understand Keys Fields - How this improve performance?

Hi guys,

i´m newbie on Nav and i had some doubts for the Fields of the Keys Table. I want to know what impact they can do the performance of NAV.

I have read this http://msdn.microsoft.com/en-us/library/dd338617.aspx.

Field KeyGroup
Explanation: Keygroups - Determines which key groups the key is a member of. By making the key a member of a predefined key group you can have the key defined and only enable it when it is going to be used.Adding a large number of keys to a database table decreases performance. However, by making the keys members of predefined key groups you can have the necessary keys defined and only activate them when they are going to be used, thus improving performance.
1 -Can you explain how adding a large number of keys can decreased performance?
2- Why understand how works Clustered Indexes an NCI. Tell me if i had a new record this indexes (CI, NCI) are updated right way? If so this decreases performance?

Field MaintainSQLIndex Property
1 -This option has always to e selected for every key ( PK or SK)?

Thanks in advance for your explanations?