No, it wasn't because of vocabulary, which has only very slowly increased over time. The reading difference is instant and very noticeable. I can't read hiragana fast enough (matching speech) to follow subtitles which are all in hiragana, for example, while I can if there's kanji (though only if I can read it, there's still lots I can't read). This can be changed forth and back and tested with sites like Animelon, for example.