'Why searching in dict.keys() is faster than dict.values()?

I was wondering why the speed of dict.keys() is much higher than dict.values() if there is a dictionary with millions of keys and values?

I was handling a file with millions of entries and searching in dict.values() was taking a huge amount of time compared to dict.keys().

Meaning:

if x not in dict.keys():

vs.

if x not in dict.values():

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