I have a lag spike (high CPU usage) when
starting any Minecraft Version from the Launcher
starting any world from a running minecraft instance
Confirmed also for macOS 10.14.6 with Oracle java: 1.8.0_77
Thank you for your information. Apparently I didn't search enough, I'm sorry. I'll add a comment to the issue you indicated, saying that this also happens on macOS.
I have a lag spike (high CPU usage) when
starting any Minecraft Version from the Launcher
starting any world from a running minecraft instance