Affects Version/s: Minecraft 18w30b
Fix Version/s: Minecraft 1.13.1
Machine Model Identifier: MacBookPro11,3 (late 2013)
Operating System: macOS High Sierra, version 10.13.6 (17G65)
Chipset Model: Intel Iris Pro (builtin)
RAM: 16 GB (2 GB allocated to the Minecraft Java process)
Recently, older-model Mac users were asked in a reddit thread to provide details regarding our experience with Minecraft 1.13. My experience so far hasn't been great, so I've been experimenting a bit.
A short time after joining a single-player world in 18w30b, my kernel_task process starts to chew up the CPU. When this occurs, the Minecraft Java process also starts consuming more CPU, and this escalates until my entire system slows to a crawl. The snapshot is effectively unplayable after 10-30 seconds (excessive block lag, continuous rubber banding, etc.).
Now, excessive CPU usage by kernel_task is not unheard of, and there are various suggestions as to both the cause and the resolution (often resetting the SMC and/or PRAM), but in this case I can correlate it directly to the Minecraft Java process. The kernel_task will continue its high-CPU consumption, even after I exit the world to the title screen, until the Minecraft Java process is exited. At that point, the kernel_task immediately returns to normal.
I don't know exactly why Minecraft is being so heavy on the core OS process. This was not (and is not) an issue in 1.12.2. Please let me know if you need me to track down further details.