Personnellement j'ai a peu près la même chose en IPv4 et IPv6 (mais je n'utilise pas de Livebox):
franck@Mercury:~:> traceroute google.fr
traceroute to google.fr (172.217.19.131), 64 hops max, 52 byte packets
1 gateway.xxx.me (192.168.66.1) 3.020 ms 0.762 ms 0.723 ms
2 80.10.235.253 (80.10.235.253) 3.157 ms 1.765 ms 2.039 ms
3 ae108-0.ncnic102.nice.francetelecom.net (193.249.215.102) 2.101 ms 2.082 ms 2.016 ms
4 ae43-0.nimar102.marseille.francetelecom.net (193.252.103.242) 5.984 ms 6.065 ms 6.032 ms
5 193.252.137.62 (193.252.137.62) 7.206 ms 8.524 ms 7.729 ms
6 81.52.186.142 (81.52.186.142) 6.184 ms 7.338 ms 6.093 ms
7 209.85.252.194 (209.85.252.194) 6.286 ms
209.85.252.36 (209.85.252.36) 6.376 ms 6.206 ms
8 216.239.58.1 (216.239.58.1) 16.577 ms
209.85.142.249 (209.85.142.249) 16.585 ms
216.239.58.1 (216.239.58.1) 16.556 ms
9 216.239.46.166 (216.239.46.166) 16.593 ms
209.85.247.194 (209.85.247.194) 27.191 ms
216.239.46.166 (216.239.46.166) 16.851 ms
10 66.249.94.79 (66.249.94.79) 16.888 ms 16.944 ms 16.917 ms
11 par03s12-in-f3.1e100.net (172.217.19.131) 16.328 ms 16.400 ms 16.271 ms
franck@Mercury:~:> traceroute6 google.fr
traceroute6 to google.fr (2a00:1450:4007:808::2003) from 2a01:XXXX:XXXX:XX66:fc76:58be:b62c:7a4c, 64 hops max, 12 byte packets
1 2a01:XXXX:XXXX:XX66::1 2.887 ms 0.838 ms 0.802 ms
2 2a01xxxxxxxxxxxxxxxxxxxx.ipv6.abo.wanadoo.fr 1.699 ms 1.573 ms 1.329 ms
3 2a01:cfc4:0:1f00::a 6.209 ms 6.148 ms 6.350 ms
4 2a01:cfc4:0:2100::5 54.431 ms 22.094 ms 6.582 ms
5 2a01:cfc4:0:2000::5 6.936 ms 6.734 ms 6.980 ms
6 2001:4860:1:1::1587:0:a 6.312 ms 8.000 ms 45.189 ms
7 2001:4860::1:0:760a 6.608 ms
2001:4860::1:0:7608 8.482 ms 17.585 ms
8 2001:4860::8:0:5e18 16.922 ms 18.465 ms
2001:4860::8:0:5e19 16.624 ms
9 2001:4860::1:0:4a3a 18.891 ms 18.737 ms 18.316 ms
10 2001:4860:0:1::66b 16.717 ms 18.712 ms 16.787 ms
11 par10s10-in-x03.1e100.net 16.221 ms 16.604 ms 16.144 ms
Je constate d'ailleurs de manière régulière (j'ai un serveur smokeping chez OVH) que le ping en IPv6 est meilleur qu'en IPv4.
Mon avis: Vu que tu prends déjà plus de 20 ms au second saut (le premier est la livebox), je suspecte que la cause soit la box, pas le réseau...