Learn Dutch: Ten slotte is de investering daarnaast efficiënt. - Finally the investment is efficient as well.

It certainly can be! You could say Daarnaast is de investering ten slotte efficiënt. But in this sentence, the speaker chose to put Ten slotte (finally) in the first position to signal the end of their argument. Because daarnaast (additionally/as well) is an adverb, it can easily move to the middle of the sentence.

Why does the verb is come before the subject de investering?

This is the classic Dutch verb-second (V2) rule in action. Because the sentence starts with the adverbial phrase Ten slotte, the verb is must take the second position. This forces the subject (de investering) to invert and take the third position.

Isn't it repetitive to use both Ten slotte (finally) and daarnaast (additionally) in the same sentence?

They actually do two different jobs here. Ten slotte tells the listener that this is the final point in a list. Daarnaast connects the content to the previous points, meaning that the efficiency is an additional benefit. In English, we often express this by saying 'Finally, it is efficient as well.'

Why doesn't efficiënt get an -e ending here?