That may be true for familiar words, but a long, uncommon word is unavoidably going to take more processing. Flashing "unforgettable" is probably fine, but I'd want a split second pause on "periodontia" or "imbroglie".
At a guess words with unique clusters of vowels would also stand out say 'Hawaiian'. Or other words like 'cameraman', 'minimum', 'consciousness', 'neuroscience', that all probably have a certain shape or density, even if they are flat.
This regex dictionary may be useful: http://www.visca.com/regexdict/ /^[aeiuocmnrsvwxz]*$/
Until then slowing down for longer words at least would definitely help. This could at least be an option if the authors aren't convinced its a good idea yet, then A/B test the amount of usage for people who use the option compared with those that don't.