[Rails] What it exactly do these do ( RubyXL::Parser.parse and Roo::Excelx.new) when opening a file?

Do they open a file and load it into memory? 

I'm asking because it seems to take a long time opening a file but after it opens looping into the variable is REALLY fast, could someone explain me or tell me where can I find an explanaition on how this sort of things work?

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