The cheap 18 quid HIDs I have in mine, when cold, give a full power flash for a small amount of time - maybe a 10th of a second, which seems to be enough for people to see (or for the car in front to think I'm plod and indicate to pull over lol) but they then drop to a dim light and do indeed take about 15 seconds to come up to full power from cold. When warm, they only take a couple of seconds to come back to full power. I've not found this to be a problem when driving through the New Forest, going to and from my mum's place. However, I'm using nightbreakers in my dipped beam which help.
I understand that not all HID kits work the same way as mine but thought it worth noting.
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