In my opinion, and practice, emergency funds should not be used for investments ever, this includes covering the losses from investing. Yes it can feel like you are not maximizing your returns on that amount of money but its job isn't to generate returns, it is your help smooth out the bumps in the road of life for you. In my experience life can punch hard and it never throws just one punch, it is usually a 1,2,3 combo. That emergency fund is merely there to help raise the chances of you standing on your feet after the vicious flurry.
If you want leverage and you are in the US or have a US based brokerage account use options. By using options your all in is basically the cost of the option and this ensures you will not ever lose more than you paid. PLEASE note the previous sentence is predicated on the golden rule of NEVER WRITE an uncovered/naked option, seriously do not do it!!!! I also use the 5% rule, which no one trade is more than 5% of my portfolio so no one trade will destroy me.
For example in crypto, market makers are commonly making 100% return. You haven't heard of these firms and they aren't interested in your money.
Market makers usually do quite well for themselves, but are capacity constrained: they can earn stellar returns on tens of millions of capital, but maybe not hundreds of millions or billions. You probably haven't heard of most of these firms and they would prefer to keep it that way.
of course if you want a public example, look at RenTec. They are unique in generating eye popping returns with such a large amount of capital. This is extremely uncommon. However generating comparable returns on 500-1000x less capital is significantly more common. These returns often don't compound tho, as they are severely capacity constrained.
But I’m a retail investor, and will almost certainly remain one— Not only do I not have the training to come up with these strategies, I don’t have the knowledge to judge the veracity of anyone claiming to sell them to me either. I’m content with my suboptimal portfolio that I don’t have to think about more than once a year, and have more important things to spend my time on.
Equities are different because their connection (however tenuous) to actual earnings gives them a very steady long-term uptrend. The hard part is just staying in during a crash so you don’t miss the recovery. Half of S&P 500 returns came on its ten best days.
I see. Would your thinking change if your emergency fund was sufficiently large but much smaller than your investable cash?
For example, let's say your rainy day fund was $10, and you have $100. You have $90 to invest. In this case, the short-term loan you're taking out could range from $0-270 (the majority of cases would not be covered by your rainy day fund).