It's not "BTRFS scanning" hangs up system loading. After removing btrfs-tools, system still hanging here for few seconds. Sometimes new info appears: "Begin: Running /scripts/local-premount".

Solution:

• open /etc/initramfs-tools/conf.d/resume
• replace RESUME=UUID=xxx with RESUME=none
• run sudo update-initramfs -u
• reboot your system

Related: https://askubuntu.com/a/1034952