Learn Spanish (Latin America): ¡Te engulliste el sándwich muy rápido!

You're right, it's not a reflexive pronoun. This is a great example of the "affective dative" (or dative of interest). With verbs of consumption like comer, beber, or engullir, adding the pronoun emphasizes completeness or intensity. It changes the meaning slightly from "you ate" to "you ate it all up" or "you gobbled the whole thing down."

Does the infinitive engullir have any tricky conjugations to watch out for?

The tú form used here, engulliste, is completely regular. However, because the stem ends in the -ll sound, the third-person preterite forms drop the i that normally appears in -ir verb endings. So, instead of "engullió" or "engullieron", it becomes engulló (he/she gobbled) and engulleron (they gobbled).

Why is rápido used instead of rápidamente to mean "fast"?

In everyday spoken Spanish, certain adjectives like rápido (fast), lento (slow), and duro (hard) are frequently used as adverbs. You can certainly say muy rápidamente, but saying muy rápido is much more common and sounds very natural in conversation.

Could I say tu sándwich instead of el sándwich if I wanted to say "You gobbled your sandwich"?