My Red Planet took forever to lay down the base, but once it started sprouting branches, it has really taken off. I also have a Blue Tip Green Slimer, Miami Orchid, and Tubs Pink Jade that grow like weeds for me, along with a couple Birdsnests. My Blue Polyp Digita, and an Idaho Grape, are fast growers as well.
As for slow growers, mine would be a Rose Mille. Has been laying a base for a year, but refuses to sprout branches. I also have a Blue Tort, a Pink and Green Loripes, and a Pink Stylophora that are taking their sweet time at growing as well.