Warn users in Web Vitals and trace view that Timing-Allow-Origin must be set for accurate scores

[bcoe]

Problem Statement

Problem

We've noticed a behaviour described here in which LCP can happen before FCP.

The theory is that this relates to the LCP tying to a cross origin asset, such as an image. We've yet to confirm with an ongoing customer case, but we believe this problem might addressed by setting the Timing-Allow-Origin header.

We believe this problem affects a healthy number of users and leads to web vitals scores that don't look trustworthy.

Solution

In the Web Vitals UX and Trace View UX, if we observe LCP < FCP, display a tip to the user that explains that setting Timing-Allow-Origin may be necessary.

Note: we should wait until we confirm this fix in the wild before we do so.

Solution Brainstorm

No response

Product Area

Performance

[getsantry]

Routing to @getsentry/product-owners-performance for triage ⏲️

[bcoe]

Blocked waiting to see if setting this header actually fixes timings.