Yesterday when I pushed the new version of Chrome 67 I got an alert on my own setup that Speed Index increased. I checked first visual change too and the same there. I rollbacked to 66 and the metrics got back to normal. Today I pushed 67 again and I could see the same thing. I could also see first paint changes. But only on Wikipedia URLs. Paul Irish pinged me on Twitter and said he could help out. I've checked our WebPageTest tests today (I haven't updated Browsertime yet) and I could see the same thing - 67 seven increased the metrics:
The original issue: https://github.com/sitespeedio/sitespeed.io/issues/2069
Since we see a change in first paint too I'm pretty sure we will see the same in our RUM metrics when 67 rolls out.