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Depends on what the instructions are. Adding salt at the beginning elevates the temperature of the water which speeds the cooking.
There needs to be enough boiling water in the pan so when the pasta is addded it resumes boiling quickly. If too much pasta is added it drops the water temperature.
Also cooking fresh pasta needs a lot more salt as it is not in the water for so long.
I thought you just bang some salt in near the end of cooking time. Am I doing it wrong?