The difference in reading speed only applies to people who are used to double-spaces, there is no difference in reading speed single-spaced vs. double-spaced for people who are conditioned to read single-spaced.
> Although the type of spacing following punctuation marks did not seem to have an effect on those individuals who type with one space after a period, those who type with two spaces after a period had greater reading speed when paragraphs were presented in the same way in which they type: with two spaces following periods and one space following commas.
I'd argue this data suggests the double-spaces are a waste of space that accomplishes nothing except slightly handicapping people who become accustomed to it and then read text without it.