Start with the conflict of interest: we make one of these apps. Treat any ranking here as suspect, including one that flatters us. What this page can honestly offer is what the category looks like from public store data, and the checks you can run yourself in ten minutes.
Scanning the App Store for single-word reading apps — those describing themselves with RSVP, Spritz, Word Runner or flash reading — turns up roughly two dozen products. The largest of them has under two hundred ratings. Most are free.
That is a small category by any measure. It means two things. Nobody has won it, so there is no safe default choice. And review counts are too low to tell you much — a 5.0 average over three ratings is noise.
It is also worth knowing that the best-known name, Spritz, exists mainly as licensed technology rather than a reading app people use daily, and that Kindle removed its Word Runner feature entirely. Some of the demand in this category is people looking for something that went away rather than shopping for something new.
Any multiplier. Comprehension holds to roughly 350 words per minute and drops measurably between 400 and 450; above 500 single-word presentation degrades understanding for everyone, more so in a second language. An app promising three times faster reading with full comprehension is describing something the evidence does not support.
Worth knowing too: when presentation formats are compared directly, showing more context at once measures better for comprehension than one word at a time. The whole category is a trade of context for focus. That is a good trade for some readers and a bad one for others, and no app can tell you which you are.
More on the evidence in does speed reading hurt comprehension, and on file handling in reading EPUB and PDF without sending your files anywhere.