The minimal fix for the sluggish calculator on Ubuntu 18.04 to 19.10:

  sudo snap remove gnome-calculator
  sudo apt install gnome-calculator

The more complete solution is entirely removing snap. I did that long ago, but my process was a little painful. From my logs, it went something like this:

  snap list
  snap remove <copy-paste packages from above>
  sudo service snapd restart # core is tricky to get rid of
  sudo snap remove core
  snap list # check it's empty
  sudo apt install gnome-calculator gnome-characters gnome-logs gnome-system-monitor
  sudo apt purge snapd squashfs-tools gnome-software-plugin-snap
  rm -r ~/snap

See also: https://ubuntuforums.org/showthread.php?t=2409173&p=13826670...