MC-179383 finally got fixed in 1.21.11, improving performance in forested areas. There is, still, however, the issue that blocks next to opaque leaves are still rendered in their entirety, which should not happen as they are not visible in this state.
To reproduce, go into Spectator mode and enter a tree with your graphics setting set to make leaves opaque.
Expected behaviour would be for the logs on the inside of the tree to not be visible. However, they are, which is wrong.
Thank you for your report!
After consideration, the issue is being closed as Won't Fix.
Please note that this is not the same as Working as Intended, as this bug report correctly describes behavior in the game that might not be the intended or desirable behavior, but it will not be fixed right now. Sometimes, this is because the issue reported is minor and/or impossible to change without large architectural changes to the code base.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support (Technical Issues) – 📧 Microsoft Support (Account Issues)
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki