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Respecto a tu pregunta, si configuras "el de Jazztel" en modo bridge será el neutro el encargado de establecer la conexión PPP, así que la latencia no debería sufrir trastorno. Si, por el contrario, configuras dos subredes, habrá una ligera pérdida, tal que así (ésta es mi red):
$ ping -c 10 linksys
PING linksys (192.168.1.254) 56(84) bytes of data.
64 bytes from linksys (192.168.1.254): icmp_req=1 ttl=64 time=1.35 ms
64 bytes from linksys (192.168.1.254): icmp_req=2 ttl=64 time=1.23 ms
64 bytes from linksys (192.168.1.254): icmp_req=3 ttl=64 time=1.29 ms
64 bytes from linksys (192.168.1.254): icmp_req=4 ttl=64 time=1.18 ms
64 bytes from linksys (192.168.1.254): icmp_req=5 ttl=64 time=1.29 ms
64 bytes from linksys (192.168.1.254): icmp_req=6 ttl=64 time=1.26 ms
64 bytes from linksys (192.168.1.254): icmp_req=7 ttl=64 time=3.56 ms
64 bytes from linksys (192.168.1.254): icmp_req=8 ttl=64 time=4.55 ms
64 bytes from linksys (192.168.1.254): icmp_req=9 ttl=64 time=1.16 ms
64 bytes from linksys (192.168.1.254): icmp_req=10 ttl=64 time=1.21 ms
--- linksys ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9013ms
rtt min/avg/max/mdev = 1.167/1.813/4.554/1.146 ms
$ ping -c 10 comtrend
PING comtrend (192.168.0.1) 56(84) bytes of data.
64 bytes from comtrend (192.168.0.1): icmp_req=1 ttl=63 time=2.84 ms
64 bytes from comtrend (192.168.0.1): icmp_req=2 ttl=63 time=1.69 ms
64 bytes from comtrend (192.168.0.1): icmp_req=3 ttl=63 time=1.75 ms
64 bytes from comtrend (192.168.0.1): icmp_req=4 ttl=63 time=1.76 ms
64 bytes from comtrend (192.168.0.1): icmp_req=5 ttl=63 time=1.65 ms
64 bytes from comtrend (192.168.0.1): icmp_req=6 ttl=63 time=1.80 ms
64 bytes from comtrend (192.168.0.1): icmp_req=7 ttl=63 time=1.66 ms
64 bytes from comtrend (192.168.0.1): icmp_req=8 ttl=63 time=3.90 ms
64 bytes from comtrend (192.168.0.1): icmp_req=9 ttl=63 time=1.90 ms
64 bytes from comtrend (192.168.0.1): icmp_req=10 ttl=63 time=1.68 ms
--- comtrend ping statistics ---
10 packets transmitted, 10 received, 0% packet loss, time 9014ms
rtt min/avg/max/mdev = 1.659/2.068/3.903/0.698 ms
Como ves, de media (avg) ha perdido 0.2 ms de un router a otro. No soy jugador, pero creo que es una merma de latencia casi ridícula. Y sería en la peor opción (la de configurar dos subredes, en caso de que no pudieras poner el de Jazztel en modo bridge).