Build and upgrade speed up

What if you can use any amount of gems to help speed up the build process? Say there's 100 gems needed to complete the build or upgrade but you only have about 20. Use the 20 gems to reduce the amount of time left on the build or upgrade so that you would then have 80 gems left To finish the build or upgrade and a lesser time.

This is redundant; I don't really see a reason to make a specific button for this. You could just wait until it's the amount of gems that you want to pay to finish the upgrade and then hit the gem option.