I'm not all that familiar with the uk sytem, but I'd imagine that it takes the same time with both. This has more to do with the various ISPs and the times at which they update their errrr... tables would it be called? Anyway, I don't think it has much to do with the number of domains, more about *when* each ISP updates (I believe generally every 12 hours, just not all at the same 12 hours, so if you miss the updates by a minute, it could add at least 12 hours right there).
Feel free to correct any of this for those who know more about it.