I think the thing that makes it a problem for most people is that they can't memorise them as "once and done" the way Heisig says he did. So as well as the initial time spent looking at the kanji and coming up with a good memorable story/image/mnemonic/etc, most people I think also spend time in an SRS (e.g. anki, or kanjikoohii) reviewing the characters they learnt previously. It's the review time that really stacks up, especially where you have particular characters that you have trouble with ("leeches").