Manually installed MCL 2.1.7658 via Minecraft.deb on Linux Mint 19.2 after purging MCL 2.1.5965. Immediately noticed higher CPU usage after clicking "Play" (both on the menu screen and while playing).
System Monitor is showing a considerable amount of kernel CPU use when Minecraft is running - this was not happening on MCL 2.1.5965.
Purged the new launcher and went back to MCL 2.1.5965 and Minecraft behaved normally again.
This issue should be fixed as of the latest launcher beta (2.1.8629), released Friday last week.