They are not particularly slow, but it depends on cultivars. Early cultivars are much faster starting than late season types. It is one of the easiest vegetables to grow and benefits from benign neglect. You may be overdoing the care, particularly the water. Good fertile soil or poor soil with some fertilizer and they are good to go.
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Mine seems to start slowly also, compared to the other plants around them. However, once the heads form, they seem to double in size every few days. I grow late (like 85-90 day) cabbage and find it to be better tasting than the earlier varieties, but that's just my opinion.