Assuming I have $100k now, what's the fastest way to double this within 5 years?

Buy bitcoin when it hits the low of its bear market

It's easy to tell how to halve it within 5 years but nobody can tell how to double it with tolerable risk and probable success 5 years from now. That would be an unrealistic 14.9% annual growth rate, impossible as a planned action without much, much luck (= chance).

I believe it depends on what that 100k is to you. If it's most of your savings, I'll be extra conservative with my investments by focusing mainly on stocks that are fundamentally strong or index funds.

Else, personally I would allocate 90% on stocks, 10% on crypto as it suits my risk appetite. imo, knowing your risk tolerance is important if you're getting into highly speculative assets like crypto. Either you multiply it or lose it all so only set aside money that you're willing to lose. As for stocks, I would tap on innovative/growth stocks (nothing more than 5, just my personal preference). The higher the conviction I have in a particular company, the higher I will allocate into it. Definitely higher risk exposure than index funds, however higher potential in achieving greater returns.

Additionally, from active income, I would just set aside an amount to compound weekly/monthly on US index funds over the years.

Just my 2 cents worth. Hope it gives you some insights in anyway. Cheers!

Work your ass off and save 70% of your salary. You’ll get more than double that amount by year 5.