How fast can you say a tongue twister without errors?
Tongue twisters achieve their difficulty through the combination of similar but distinct phonemes that require precise articulatory positioning — positions the mouth must reach and differentiate rapidly in sequence. The most effective tongue twisters exploit the minimal phonemic pairs most difficult to distinguish at speed in a given language.
In English, the recurring difficulty sources: the r/l distinction in "red lorry yellow lorry" (though this pair is easier for native English speakers than for speakers of languages without the distinction), the sh/s distinction in "she sells seashells," the p/b plosive distinction, and words that require rapid alternation between lip-rounding and spreading vowels. The challenge is motor-programming — your mouth must execute a precise sequence of muscle movements without the slight time buffer that normal speech speed provides.
Speed test performance on tongue twisters correlates moderately with overall speech articulation speed and verbal working memory — making it a mildly interesting indirect measure of verbal fluency, in addition to being entertaining.
Related: Typing Speed Test · Reaction Time Test