It's actually faster to read a manga than it is to watch an anime. In a manga, there is little to no filler and you read at your own speed. Anime on the other hand, is predetermined time and may have a lot of crap in it that you've seen once, twice, three times etc.
When you start cutting out opening scenes, ending scenes, previews, recaps, flashbacks, and fillers.... You're left with about 10 minutes of content in a 23 minute video. Sucks, but it's truth. And that's still slower than reading the manga. Typically.
Yeah I've been told that, but I don't know I just like anime better than manga. I guess I just like the sound and visuals better, I get more from the anime than I do from manga. But I think it's just me.