In the early stages of the game it is advised that you use the Gems that you earn towards upgrading your VIP level as doing so will give you long term benefits such as increased building and research speed and also an exclusive daily chest with VIP level appropriate rewards.
Once you reach VIP 6 you will permanently unlock your second builder. After VIP 6 you will not get a third builder if you use 2-day builder scrolls but instead those scrolls will be converted to speed ups.
When you reach the later stages of the game you may want to consider spending the Gems you earn on purchasing speed up items from the courier station where you will find they are heavily discounted.
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