Realtime feedback on TimeToLargestContentfulPaint

[shipify]

In my app's admin performance, it indicated TimeToLargestContentfulPaint (LCP) was too long. And the data provided by Shopify has 2 days of lag, i.e. today is 26th, but it is showing stats for 24th.

It also gave no indication of what could be causing this high LCP.

How do I get realtime feedback? Otherwise it is impossible so solve this, I have wasted 1 week on it already.