On a new Ubuntu 18.04 install, several applications seemed to take a very long time to open.
These are Snap apps.
First, open a Terminal and remove the Snap versions of the apps:
sudo snap remove gnome-calculator gnome-characters gnome-logs gnome-system-monitor
Second, install the native, non-Snap versions of the apps:
sudo apt install gnome-calculator gnome-characters gnome-logs gnome-system-monitor
Optional, remove Snap:
sudo apt purge snapd