The G sound is crucial. D only exercises the top muscles of your tongue. The G pulls it back down, so both the up and down stroke are firm. Plus using two muscle groups means half the fatigue in each.

As for the speed: You’ll get faster. This is something musicians call “double tonguing” and it takes a while to learn. But trust me, it’s worth it.

And if you’re down for more self improvement to make your very lucky wife happy, work on increasing how long you can hold your breath. That really helps for those “Oh god don’t stop” moments that seem to last forever.