it's not really a matter of them being slow growers, it's just that they max out at about 4". that being said, they don't climb, and as such, are limited to finding food on the substrate or low on the tank walls/rockwork, as they can only feed as high as their proboscis can reach.
ideally, you should have 2 square feet of substrate to support one.
IME, they do go thru periods of inactivity, and sometimes bury themselves in the substrate for extended periods as well.