The most popular IETF standard for VPNs is IPsec using ESP.
IPsec could be improved for wireless:
Now PPP-over-SSH isn't any better, just a lot easier and much less
expensive for most people on both Unix and Windows. Either would
probably be an acceptable mobile VPN solution. IPsec would probably
add less latency, but not much compared to what existing wireless
customers are used to.
If I ever get 128kbps Ricochet on my neighborhood lamp-post, I'll
try and see what the round trip time for ppp-over-ssh ends up. I'm
not thrilled with the 0.7s first-hop round-trip of plain Richochet
wireless service to begin with, nor with the fact that the location
of that first hop is New Jersey when the modem is in California.