Yes, check the Session Management code. It's showing as code, so verify how they are being managed.
Also, take a look at compilations. There are metrics for that, but you can also check "Code level" in the distributed traces and see them at the beginning of the requests. Had several problems with PHP/Symfony compilations in the past.
I already try Code Level on some traces, but only Other time is shown.
How to deep dive ?
We tried changing disk where sessions are stored (network disk before, local disk now), but nothing change in response times.