It's about as purely Esperanto as you can get. -e is the adverb ending. -et- is a suffix indicating a reduction of the root. Iom is one of the 45 correlatives, which are built from one of 5 beginnings and 9 endings. i- indicates a non-specific, and -om indicates a quantity.
This is the full table of correlatives: https://en.wikipedia.org/wiki/Esperanto_vocabulary#Correlatives