This is absolutely true! Having more add-ons and toolbars means that your processor is going to be working much harder to function it all, while also trying to handle everything else it is getting. If you have to many add-ons/toolbars, it is going to be a constant fight on the IRQ and with a lower-end PC, your RAM may even fill up and crash your computer.
To put it simple, only add the toolbars and add-ons you need, and don't add several extras.