"Я съел кашу сразу, а апельсин — через час."
Translation:I ate the porridge right away, and the orange an hour later.
Я думаю "а" можно перевести и как but. Присутсвует какая-то доля противопоставления.
Not a native speaker but shouldn't we use the future in the second part of the sentence ? Like "I ate the porridge right away, and i will eat the orange an hour later"
i think your suggestion highlights why "i ate the porridge right away and the orange in an hour" is not only awkward, but probably wrong! If "i ate the porridge right away and the orange one hour later", then i would understand this as both having taken place in the past (e.g. porridge 2 hours ago and apple 1 hour ago). But "in an hour" is in my understanding used for something which will happen in the future (e.g. porridge 6 minutes ago, apple in one hour). But the Russian sentence has only one verb, and it is in the past tense. And maybe worth noting the verb isn't only in past tense, but perfective as well.
As a native speaker the meaning I get from "I ate the porridge right away and the orange in an hour" is that it was done in the past, within an hour of finishing the porridge. Compare with "I did my homework in an hour".