[plt-scheme] eqv? equal?

Sorry for this newbie question.But i wonder so much about eqv? and equal?.In
the some discussin that passes on ffreenode #scheme.I see some sentence
which is "eqv? is faster than equal? in comparison.So i thought eqv? and
equal? returns boolean thing #t or #f right ? So why?

In that point,i have three question about that.

what is the thing that makes eqv? faster than equal?.

which one is more powerful on programming  ?

What is the reason that equal? still be used ? If eqv? is faster than
equal?.

Regards.
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