Validation of routing policy to ensure others aren't abusing you (pointing default, for example). As for orders of magnitude, once an IP option is in a packet, the damage is essentially done, otherwise looking up the path to an address in the options is no more impactive than looking up the address in the original destination field.
Well, no. Not really. First off, following the 80/20 rule (or in this case 99.x/(100-99.x) rule) says that hardware implementations which get optioned packets punt them to software. This is at every hop. Second, the IP source route is a stack of IP addresses, which must be modified at every hop. This implies not just software forwarding, but also significantly more work than an IP lookup. eric
source-routing only has security implications to those with defenses which permit traffic through some type of backdoor. The backdoor has more security implications than the source-routing, since it may be compromised in other manners.