OP, highest ROI does not come from investing in coins for a few days. It comes from patience and perseverance. Look at the price charts of bitcoin and ethereum (at least) and see the ROI of these coins and the ROI period for that specific rise/fall. Bitcoin have come from 100$ at one time to 6k+$ today even in the bear market. Do you consider that as "High" ROI? You may not because the term "High" is again relative. 
The returns that Bitcoin and a few other coins have shown are indeed quite handsome, but as you just said, it is all relative. To make viable comparisons we should also include the amount of risk associated with an investment and what category this investment belongs to. From my perspective, Bitcoin was like a startup in its early days (while some would say it still is) but so many startups go bust and bite the dust within a year or so, so amount of risk is extreme in the field. On the other hand, the returns are comparable to those that successful startups show within approximately the same time span.
Now you recall the proverb, if it looks like a duck, swims like a duck, and so on.