Problems of slow test:
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If running your tests takes a non-trivial amount of time, you’ll be more reluctant to run them. 
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If tests start to become a chore, then we’d be more willing to move on to new tasks without first finding out if there are bugs in what we just did. 
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If tests run in lengths of minutes. While we watch the test suite we are slowly forgetting what were just working on. 
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Long tests cause stress. 
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Stress leads to anger. 
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Anger leads to hate. 
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Hate leads to suffering.