In forex I have seen some people turn 300USD accounts to 5000USD in 1 month, if not 2 weeks but this was during 2008 subprime crisis.
Now to do this, they needed 1:500 leverage. Highly risky. Per trade they were risking a lot. This is no fun.
I recall giving them 40k to trade and they turned 40k balance into 15k balance. 25k loss. The trader could not get his head around big money he was trading. Plus getting filled was difficult and 1:500 was not offered.
So anyone who claims to make 20%-40% a month is full of lies. You can do that with a tiny pocket money account with 500usd but when you give them big amount of money, they you could say bye bye to your money.
Tight risk management with system based rules are the way forward.
You should be looking for 20% to 40% on average profit per year. This is what is expected from forex.
After so many years of wasting money here and there, I have finally stuck with my trader from Europe who has been making me around 60%-80% per year net. There are drawdowns sometimes of around 5-10% during the month. But that does not matter as the system based strategy has been making consistent profit.