You are right about that, at least according to that article here:https://news.ycombinator.com/item?id=18942572
That guy trained on a well known website to solving minor coding issues and then (that's the important part) practised solving them in mock interviews to real persons.
Maybe that could help you.
You can practice all you want to get better, but no one in the corporate world is saying "I need you to code bubble sort, and I need in the next 45 minutes" and then proceeds to stand over your shoulder the entire time.
As for wasting a lot of time, what more reliable method is there that takes less time than a leetcode-style problem? Surely there are more reliable methods, they probably don't take 30 minutes or less though.