That's correct, since the Grandmaster increases your "profession speed" by 50%. It doesn't offer 50% reduction in time required.
Think about it this way: if you have to drive 30 km @ 10 km/hr, it would take you 3 hrs. If you drive 50% faster (15 km/hr), it would take you 2hrs (a 33% reduction in total travel time).
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Think about it this way: if you have to drive 30 km @ 10 km/hr, it would take you 3 hrs. If you drive 50% faster (15 km/hr), it would take you 2hrs (a 33% reduction in total travel time).