Internal interfaces are faster than physical interfaces. Their speed depends on the FortiGate unit CPU and its load. That means that an inter-VDOM link interface will be faster than a outbound physical interface connected to another inbound physical interface.

Inter-VDOM links are CPU bound, and cannot be part of an accelerated pair of interfaces.

However, while one virtual interface with normal traffic would be considerably faster than on a physical interface, the more traffic and more internal interfaces you configure, the slower they will become until they are slower than the physical interfaces. CPU load can come from other sources such as AV or content scanning. This produces the same effect—internal interfaces such as inter-VDOM links will be slower.