Perfomance of PCIe expander?
There's different things that can be called expanders:


Lane bifurcators
PCIe switch-based backplanes


Lane bifurcators

A PCIe socket can have more than 1 PCIe lane – for example, x4 and x16 lane slots are pretty common. If the root complex and all PCIe switches in between support it, these lanes can either be used for only one link, giving more bandwidth, or be split among different PCIe devices.

If that is supported, then a basically passive card could be used to e.g. connect four x1 sockets to a x4 socket. Again, this is a special case and needs support from the hardware. That technique is called "lane bifurcation".

Obviously, that means the overall bandwidth is split according to the number of lanes that the end devices get.

Full switch-based backplane

PCIe is a point-to-point link system.

Hence, just like in Ethernet, you can have switches that add more links to a single link. Of course, the devices sharing that link can not magically get a higher sum bandwidth than that original link offered, but the bandwidth isn't statically alotted – just as in ethernet, each device will get as much bandwidth as it wants until the bottleneck gets congested.