I just checked the laptop and the /proc/mounts line for the portable drive is
/dev/sdb1 /run/media/andre/Seagate btrfs rw,seclabel,nosuid,nodev,relatime,space_cache,subvolid=5,subvol=/ 0 0
and indeed there is no sync! (Otherwise it's identical to the one posted above from my desktop.) I realized that I have cinnamon installed on my desktops, but not the laptop. Now I have to look into disabling sync, hopefully that will fix it. Thanks for the link!