Note that US PhDs are a longer process, you will typically have substantial teaching obligations, which are the basis of your stipend and take time away from research.
Whereas in the UK you’ve done more in depth work at undergrad (hence 1 year masters instead of 2) and the PhD is what in US terms would be called a fellowship (ie teaching is not required, though a bit may be done for extra money) with funding strictly limited to 3 years.
Taking 6 years for the US masters plus PhD is very common, and some take much longer, because the teaching stipends are somewhat open ended (you are cheap labor for colleges). What supervisors really don’t want is for you to take the stipend then bail after the masters, but it certainly does happen, especially in a lucrative subject like CS.