While this was true in the past, it is not so today. In the early 1990s, adding packet filtering to a Cisco Systems router could cut throughput by as much as 70 percent. Today, routers have better designs, and it is possible on some routers to block packets with spoofed source addresses with no effect on throughput at all. Using the same mechanism, a router can examine the source address and determine if it came from the correct interface. If the route is not the same, the source address must be spoofed, unless asymmetric routes are being used.
Asymmetric routes mean that there is more than one way to reach the destination.
If asymmetric routing is not in use, enabling this facility will block all packets with spoofed source addresses. Terminal servers can also block packets with spoofed source addresses. Some terminal servers do this by default. The simple solution is to block packets with obviously spoofed source addresses from entering your network. Most firewalls do this by default.