Here is a code example, not from my code base but from monk tybalt89's response to one of my earlier questions. The problem is generic enough.
Noticable in this example is that the code up till Mainloop is actually quite fast, but then it takes a couple of seconds until the whole GUI is drawn.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11136705
use warnings;
use Tk;
use Tk::Pane;

my ($rows, $columns) = (70, 20);
my $sbsize = 22;

my $mw = MainWindow->new();

# example of ->pack in a different Frame

my $buttonbar = $mw->Frame(-bg => 'blue',
  )->pack(-fill => 'x');
$buttonbar->Button(-text => 'Exit', -command => sub{$mw->destroy},
  )->pack(-side => 'right', -fill => 'x', -expand => 1);
$buttonbar->Button(-text => 'Set Size', -command => \&setsize,
  )->pack(-side => 'right', -fill => 'x', -expand => 1);
$buttonbar->Button(-text => 'Remove Column',
  -command => sub{$columns--; changegrid(); setsize(); },
  )->pack(-side => 'right', -fill => 'x', -expand => 1);
$buttonbar->Button(-text => 'Add Column',
  -command => sub{$columns++; changegrid(); setsize(); },
  )->pack(-side => 'right', -fill => 'x', -expand => 1);
$buttonbar->Button(-text => 'Remove Row',
  -command => sub{$rows--; changegrid(); setsize(); },
  )->pack(-side => 'right', -fill => 'x', -expand => 1);
$buttonbar->Button(-text => 'Add Row',
  -command => sub{$rows++; changegrid(); setsize(); },
  )->pack(-side => 'right', -fill => 'x', -expand => 1);

my ($screenw, $screenh) = ($mw->screenwidth, $mw->screenheight);
print "w $screenw h $screenh\n";
$mw->maxsize($screenw, $screenh);

#$mw->maxsize(1000,1200);
#$mw->minsize(400,300);

my $scroll = $mw->Scrolled('Pane',
                         -scrollbars => 'osoe',
                         -sticky => 'news',
  )->pack(-expand => 1, -fill => 'both');

my $f = $scroll->Frame->pack(-expand => 1, -fill => 'both');
changegrid();
$mw->update;
setsize();

MainLoop;
-M $0 < 0 and exec $0; # FIXME for testing

sub setsize
  {
  $mw->update;
  my( $mwh, $sh, $fh) = map $_->height, $mw, $scroll, $f;
  my( $mww, $sw, $fw) = map $_->width, $mw, $scroll, $f;
  my( $mwrh, $srh, $frh) = map $_->reqheight, $mw, $scroll, $f;
  my( $mwrw, $srw, $frw) = map $_->reqwidth, $mw, $scroll, $f;
  my $deltaheight = $sh - $frh;
  my $deltawidth = $sw - $frw;

  print "height $mwh $mwrh $sh $srh $fh $frh\n";
  print " width $mww $mwrw $sw $srw $fw $frw\n";
  print "delta h $deltaheight w $deltawidth\n";
  my $geo = $mw->geometry;
  my ($w, $h) = split /\D/, $geo;
  my $newgeo = ($w - $deltawidth + $sbsize + 10) .
    'x' . ($h - $deltaheight + $sbsize);
  print "old $geo new $newgeo\n";
  $mw->geometry( $newgeo );
  }

sub changegrid
  {
  $_->destroy for $f->children;
  foreach my $row (0..$rows - 1)
    {
    foreach my $col (0..$columns - 1)
      {
      $f->Entry(
        -text => "rc $row $col",
        -width => 0,
        )->grid(-row => $row, -column => $col, -sticky => 'news');
      $row or $f->gridColumnconfigure($col, -weight => 1);
      }
    $f->gridRowconfigure($row, -weight => 1);
    }
  }
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