Significantly slower. In fact mixing slots like that you would end up with worse performance than using two 8x PCI-E 1.0 slots (which have the same bandwidth as 4x PCI-E 2.0 slots). Of course if you have a 16x and 4x PCI-E 1.0 slot the results are a little worse. Anyway, the crossfire drivers really don't like uneven PCI-E bandwidth so I wouldn't recommend it. You should take a look at this article on legion hardwarehttp://www.legionhardware.com/document.php?id=761
And you search you can find similar articles on other sites.