https://smabie.github.io/posts/2019/10/04/vol.html
It involves deriving “perfect” leverage ratios and talks about some other interesting (imho!) stuff.
Expected Return/Expected Variance
For example even if the expected return is 1% and the vol 5%, the ideal leverage ratio for maximizing return is 4x!
In short, a 2-3x leveraged ETF is an excellent investment and should outperform the index in almost all market conditions. It’s when your leverage ratio goes over 5x that you start to have major problems a lot of the time.
All "leveraged" ETFs (to the best of my knowledge) are synthetic - they achieve their "leverage" using derivatives, not by borrowing. These derivatives are not free, and like an option, can expire worthless. That's how the value in these ETFs evaporates over time, regardless of how the market performs.
If you look at UPRO, its daily returns almost exactly track 3x of SPY. There’s no long-term “decay”, unless you are referring to volatility drag. VIX etfs are the notable exception, in that they do suffer from persistant negative carry.