Ticket #31793: 31793.2.diff

src/wp-admin/js/customize-controls.js

@@ -582 +582 @@
                                         section.closeDetails();
                                 }
                         });
+
+                        // Pre-load the first 3 theme screenshots
+                        api.bind( 'ready', function () {
+                                _.each( section.controls().slice( 0, 3 ), function ( control ) {
+                                        var img, src = control.params.theme.screenshot[0];
+                                        if ( src ) {
+                                                img = new Image();
+                                                img.src = src;
+                                        }
+                                });
+                        });
                 },
 
                 /**