one more solution you could consider if you want to improve performace (this is what I use):
It's better to know that old navigation is going to die, then not to know. Now I'm sure I have to migrate to self designed navigation. To be able to make good decision wich way to choose it would be good to know time frame of to-be-developed vertical tabbed pane. This vertical tabbed pane is propably way we will go - but still it depends o its avaiability. Even information that it will be not faster then...... is better then no information.
For best results, use the Firefox browser..