For programs that require large load times, it's a common practice to
display a Splashscreen that occupies the user's attention.  This
Toplevel mega widget provides all the display, destroy and timing
events.  All you do it create the Splashscreen mega widget, populate
it as you see fit, then invoke Splash() to display it and Destroy() to
tear it down.

WWW: https://metacpan.org/release/Tk-Splashscreen