I have a few colonies of Dendros myself and to me they are extremely fast growers. I can literally see them budding right in front of my eyes!
However, they are always hungry and need to be kept fed to stay healthy. And the more you feed it, they more they need it. It's almost like giving drugs to a drug addict. If you don't, they will just decalcify and waste away.
IMO, they are difficult corals to keep due to the fact the amount of food they need. But if you have the leisure, they will not put your efforts to waste. They are just soooo beautiful...
This one colony of mine has over 20+ polyps big and small.
