There are some slight alignment with some version releases as appeared in say beta version comparisons:
https://uma.googleplex.com/timeline_v2?sid=4a9ad886df8addcf1b4bf29f16e70302

The problem seems to be coming from multiple sources. However, to narrow down the problem I only focus on the regression across all the channels. Looking at UKM, across all the top websites it seems Youtube is the problem. Although the amount of data is a lot less that what we have in UMA. Anyhow, the youtube latency got almost doubled there.
I couldn't find the exact date they rolled out the new UI. Anyone knows that?

I wonder if any of this data is actually valid given that touch scrolling is broken in Chrome 61 on Windows. See  issue 747642 

Re #3: we're confident this is unrelated.  We can see the metrics get worse across all channels right around the days when Polymer YouTube rolled out.  Root cause is probably https://github.com/Polymer/polymer/issues/3604 (YouTube used to scroll without blocking on touch listeners, with polymer it now blocks).