As with attention, most search results here are aimed at children and at teaching reading. This page is for adults who already read and want the process to cost less.
The same caveat applies: an app is a tool, not a treatment, and nothing here diagnoses anything.
Two typefaces are commonly offered for dyslexia — OpenDyslexic, with weighted bottoms intended to reduce letter rotation, and Atkinson Hyperlegible, designed for low vision with letterforms that are hard to confuse. We ship both.
What we are not going to tell you is that either is proven to make you read better. The research on dyslexia-specific typefaces is contested; several studies find no reliable advantage over a good standard font, and preference varies a lot between readers. Offering the option is reasonable. Claiming it is a fix is not.
Letter and line spacing is a more mundane setting and generally a more useful one. It is also the setting most apps bury. If you try one thing, increase letter spacing before you change the typeface.
Related: display duration that varies by word length measurably improved reading rate in readers with central vision loss. Word length is not the same variable as dyslexia, but it points the same way — the timing of the presentation matters at least as much as the shape of the letters.
RSVP removes the line. No line means no line to lose, and no crowding from neighbouring words. If your difficulty is tracking across a line, that is a direct benefit.
But when presentation formats were compared for comprehension, formats that show more context scored better than one word at a time. If your difficulty is holding the sentence together as it goes, more context — not less — is what the evidence supports. Guided reading, where a highlight moves over normal text, is the format that measured best.
See also RSVP reading with ADHD — for adults and what RSVP reading actually does.