Hello there,
This is what I’m getting from my tests: https://share.getcloudapp.com/WnuZ0Pd6
These are lab results.
The LCP issue is present in your Field results though.
If you just switched to GeneratePress. The Performance score won’t reflect immediately over to the Field results. They take around 28 days for the Performance scores to cycle.
With that said, if it hasn’t passed 28 days since you switched to GeneratePress and finished your layout changes, it would be good to wait for 28 days and recheck the field data. For now, as per the lab data, your current performance rating is quite good.