Grade level and reading ease will also impact skimming. Speed-reading-friendly direct mail scores around the 6th-8th grade level. If a letter I'm writing scores above 8th grade, I automatically rewrite and lower my score.
The Flesch-Kincaid grade level for your letter is 10, which is around Wall Street Journal country; while its Flesch reading ease score is 54. Neither is stellar for a piece of supposed correspondence, although the reading ease score is low-acceptable.
Grade level — and the speed with which I can consume your text — are directly related. The lower the grade level of your letter, the faster I'll read it.
Grade level has nothing to do with vocabulary. A robot grades your prose in Flesch-Kincaid; it doesn't understand what you're saying. It's only looking at the ratio of short to long: words, sentences, paragraphs. Shorter is faster. Low grade levels are faster.