At low levels I just make 1 of everything on the crafting list for the first time bonus, and process some extra Lumber\Ingots\Hides.
This way even if you have extra stuff you won't use, you can always sell it on in the market.
I can't speak for what it is like at higher levels where I expect it will slow down quite a bit.
Manually crafting and trying to get the quality of the item up nets much more exp than a quick synth too.
This is why I only quick synth things I am already getting little exp for.
You also should always have a food buff, even the crappiest foods give an extra 3% exp.
The culinarian in the market ward should sell a nice cheap +CP food that is great for crafting.