So based on some comments above I've gotten some ideas:
- just crossing the bid-ask spread could easily reach .5% which rules out any strategy involving holding for only a few monthsvery short periods
- based on my naive and simplistic spreadsheet backtesting just looking at SPX over the last 5 years, which for better or for worse contains the 2020 COVID roller coaster:
- for 1 month holding periods, 32% had negative nominal returns, the worst was -49% return and the mean was .9% (11.1% annualized)
- for 3 month holding periods, 32% had negative nominal returns, the worst was -%2828% return and the mean was 3.6% (14.3% annualized)
- for 6 month holding periods, 28% had negative nominal returns, the worst was -%2525% return and the mean was 7.4% (14.8% annualized)
- for 9 month holding periods, 29% had negative nominal returns, the worst was -%2121% return and the mean was 11.6% (15.5% annualized)
So my conclusion is that these are all pretty risky in that they have a good chance of being significantly negative. On the other hand all the typical returns are way above the best high-interest savings account.
I'm still pretty unsure if I'm missing any other factors.