Does someone has any suggestions how to make 17 BTC with 7 BTC starting capital in short period of time?
No investment is guaranteed to generate you profits in a short time. The only way is gambling, or trading in BTC or in another currency (but it's basically gambling as well).
I agree with you, all investments are taking time before it generates profit. And if you want to invest and wants only to get profit for a short time then for sure you are going to get small profit only. And with 7 btc for sure before it is going to grow to 17 bitcoins it is going to take 1-3 years before you are going to earn from that.
Or else you need to hard work for your own to earn without investment here at the campaign, mostly those site who promise good return turn to iscam If you aware with that stay away, if you don't want to lose your money in just click of an eye.