Trimoraic diphthong

RGEO/63

Of the “long diphthongs” ae, oe, au, ui, only ae and ui appear. … The first vowel (a and u) was in both somewhat prolonged. These diphthongs were thus of a length more or less equal to the time occupied by two syllables, and are therefore occasionally employed metrically where the normal metre requires two.

Even though in this instance Tolkien discusses Sindarin, the analysis of his recordings, as well as rhythmic lengthening triggred in two-syllable words containing ui, indicate it is likely relevant for Quenya as well.