Hi @rodyb,
We don’t support measuring Webvitals by tags yet. You might follow this issue for updates about this feature. You might also want to do this manually for now, please check out this answer.
The drift between Google and ours is mostly reflected in the LCP metric. The other Webvitals are more or less similar. The test execution environment can make drastic changes. We recommend focusing on the relative trend values rather than the actual values.
Thanks!