Well, this is how Trainer level works:
It is a very big difference from level 1 to level 2, big difference from 2-3, pretty big difference from 3-4, not that big from 4-5, small difference from 5-6, and very small for 6-7.
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A level 7 trainer is considered a luxury and would not be that different from a level 6 trainer. I would say it is definitely worth to have at least a level 4 trainer as long as you can afford it. And to explain how your players seemed to have trained faster, they could have been Very close to popping (had a high sub-level) when you trained with the Level 1 Trainer. Make sense?