Bottle nipples medium or fast flow?

My 10 week old uses Nuk bottles. Slow flow was too slow so we've been using medium flow for 2 months. When do you switch to fast flow? I think feeding would be quicker with fast, but I am not sure what is best for baby (I'm a new mom). Thanks for the help.
Update: Thanks to all for offering advice, except for those who criticized me, FYI, she takes up to an hour to feed sometimes, that is why I am considering fast flow. You should not be so quick to judge.
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