tricky performance fix

context.js

            // reloop to get proper height
            return $.when.apply($, defs).pipe(function () {
                var i, obj, node, top,
                    //isVisible is only needed in for loop; visible selectors are slow, avoid them if possible
                    isVisible = $i > 0 ? container.is(':visible') : undefined,
                    height;
                for (i = 0; i < $i; i++) {
                    obj = labels.list[i];
                    obj.top = cumulatedLabelHeight + obj.pos * itemHeight;
                    node = labels.nodes.eq(i);
                    node.css({ top: obj.top + 'px' });
                    height = (isVisible && node.outerHeight(true)) || labelHeight;
                    cumulatedLabelHeight += (obj.height = height);
                }
                // add tail?
                if (options.tail) {
                    tail = options.tail.call(self, all.slice()) || $();
                    top = all.length * itemHeight + cumulatedLabelHeight;
                    container.append(tail.css({ top: top + 'px' }));
                    cumulatedLabelHeight += tail.outerHeight(true);
                }
                node = clone = defs = null;
                return cumulatedLabelHeight;
            });

fix.diff

diff --git a/ui/apps/io.ox/core/tk/vgrid.js b/ui/apps/io.ox/core/tk/vgrid.js
index 217d7ec..2f6ba4d 100644
--- a/ui/apps/io.ox/core/tk/vgrid.js
+++ b/ui/apps/io.ox/core/tk/vgrid.js
@@ -463,7 +463,10 @@ define('io.ox/core/tk/vgrid',
             }
             // reloop to get proper height
             return $.when.apply($, defs).pipe(function () {
-                var i, obj, node, top, isVisible = container.is(':visible'), height;
+                var i, obj, node, top,
+                    //isVisible is only needed in for loop; visible selectors are slow, avoid them if possible
+                    isVisible = $i > 0 ? container.is(':visible') : undefined,
+                    height;
                 for (i = 0; i < $i; i++) {
                     obj = labels.list[i];
                     obj.top = cumulatedLabelHeight + obj.pos * itemHeight;