As ever, you can make a minimal repro of a problem and file a bug with Google directly. I just checked again and in Chrome, for me, sbperftest's jank meter is locked on 0ms, indicating perfect v-sync. I presume there is something device-specific about any jank issues, and v-sync is implemented in the Chrome browser anyway, so it makes more sense to cut us out of the process and take it up with them.
I'd like to see some screenshots of people going through a round to see what the max jank is for each user, then also post their monitor refresh rate, OS and specs.
Would you happen to have a 144hz monitor Ashley ?
--edit : The refresh rate might not be relevant.
Tried the test with a laptop and it ran once through with showing 0 on the jank, then updated chrome from 61 to 62 and it started showing random 16ms janks here and there. The change after update might be a coincidence tho.
Desktop i5 GTX960, 60hz, Win10, Chrome 62 - Random jank throughout the test
Laptop Zenbook i5 GTX940, 60hz, Win10, Chrome 61 - No jank on the test, Chrome 62 - Random jank.
Laptop MacBook Air i5 Integraged HD 5000, Chrome 60 - No jank on the test
Will try to cross reference each of these, but start by trying NW.js with chrome versions 60 and 61.
Is there a capx for the sbperftest?