"These teachers knew better than any one else how much valuable time was wasted in getting by heart long lists of words."
This is perhaps an interesting argument for simplifying spelling. Namely to give en-US children a competitive edge on the world education scale by increasing the amount of time that is able to be devoted to other subjects.
For most of them English is a second language and thus they have to deal with the spelling on top of the difficulty of dealing with a foreign language. I think that makes it so English spelling is a bigger hinderance for them than it is for students where English is their first language (such as the majority of the US), and so spelling reform would help them more than it would help US students. In terms of competitive edges, spelling reform would lessen the edge of US students.
Of course, that doesn't mean that would be bad. As you note, less time on spelling is more time for other subjects. If spelling reform led to US students getting, say, 1% better in science or math, and led to foreign students getting 2% better, yes that worsens the US position relative to the foreign students, but looked at another way everybody improves.
What negative effects do they suffer if they misspell something?
Foreign students are much more likely to actually produce something unintelligible (regardless of spelling!), but also much more likely to be unaffected by it because they're not trying to communicate anything of any importance.
Earlier learning has amazingly little effect on later outcomes. Half of Europe have all play all the time kindergarten with almost non-existent academic instruction. Half of them have academic instruction. If you compare academic achievement of communities on opposite sides of borders but with very different numbers of years of education the differences are tiny.