Hi,
Its not at all shocking.
Let me give you an example. There are 100 cats. If you have to find total no of cats then obviously it will be a quick response. And if you have to find no of white cat then it will take time to you to find it out.
In the similar way, by putting a where clause you are asking SQL to fetch selected records.
I hope you got it.
Also,
Go Through this[
^]
Cheers