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.
As I said, once the option is in the packet, the damage is done. If the performance sucks for the person using the source-routing, who cares, assuming packets without IP options are forwarded without delay. If I'm not mistaken, most (if not all) vendors still punt the packets with source-routing options to software, even if they end up dropping the packet due to administrative decision.
eric