Reason: Whonix-Gateway VirtualBox by default comes with 768 MB RAM.
(Whonix-Workstation with 2048 MB RAM.)
That works great most of the time but Whonix-Gateway happened to freeze during apt (kernel) upgrades due to low RAM. Adding more default RAM to Whonix-Gateway wasn’t an option since that would break Whonix by default for many users who don’t have that much RAM.
As solution swap-file-creator was created. If there is less than (default, configureable) 1950 MB RAM, it will create an encrypted swap file. If more than that, it will not create a swap file. Swap file creation at boot takes some time. This makes Whonix-Gateway boot slower than Whonix-Workstation.