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.