I disabled the YouTube app and disabled NFC.. last Friday .. I've had no further data usage since then attributed to Xunlei .. I'll give it another day and then switch on either / or ...
I've seen no more usage since 4.10.18
and YouTube usage has gone back to its normal data usage.
maybe xunlei is no longer being used....
Give us a shout if you notice this in future and we'll look into it.
I am also leaning towards youtube as the main data sink. I didn't stop using it-- But here are some tips. Make sure in the settings you turn off the higher definition option, wasted data there--on our phones we can't really tell the difference. And using a power saving mode that limits application access to data when not in use is also another good option to reduce data use.
I can tell you with reasonable confidence that this issue is directly related to the YouTube app. It appears that Google, which owns YouTube, also owns a stake in Xunlei. They likely are using Xunlei infrastructre in the YouTube app, but the issue is that Optus is not correctly categorising the data usage.
I deleted the YouTube app for a couple days and, voila, no more Xunlei data usage.
I initially contacted Optus, but true to form, was fobbed off to their overseas call centre and was told that 1) they had no idea what was going on with Xunlei; and 2) I should do a factory reset (which was the wrong "solution"). I didn't do a factory reset and am glad I didn't waste my time.
Finally, in closing, I've seen a lot of alarmist posts on this thread, but please don't worry or stress over it. Your phone has not been corrupted by malware so you can sleep well at night knowing that. I don't think it's anything to do with NFC or your phone's manufacturer either. You will probably waste a lot of time and energy trying to delete all your apps and do a factory reset, but I think your first port of call should be to delete YouTube for a day or two and see if the data usage stops. Also, perhaps do not click on any embedded YouTube videos on websites which might also potentially register Xunlei usage (not sure, but possible).
Hope that helps.