Excuse my English, but I have this same issue. IPv6 is fast, IPv4 is incredibly slow.
I've never had any issues with my Ziggo connection until the recent maintenance. I had the high error rate and interference around 800 MHz on the downstream that a lot of people here seem to have had, a technician came out and replaced the AOP, but now the IPv4/IPv6 issue remains.
I can watch 4K Netflix or YouTube no problem but http downloads will go as slow as 10kb/s or timeout.
Mark Ziggo here is a traceroute for Twitch and Youtube:
Tracing route to youtube-ui.l.google.com [2a00:1450:400e:80b::200e]
over a maximum of 30 hops:
1 5 ms 3 ms 5 ms 2a02:a212:a081:1480:5667:51ff:fee4:d2f1
2 12 ms 13 ms 14 ms 2a02:a204:0:541::1
3 12 ms 14 ms 13 ms nl-rtm02a-ra1-bundle-ether2006-601.v6.aorta.net [2a02:a200:40:82::1]
4 18 ms 26 ms 18 ms asd-tr0021-cr101-lo10.core.as9143.net [2001:b88::159:102]
5 28 ms 14 ms 13 ms 2001:730:2200::5474:8082
6 19 ms 16 ms 13 ms 2001:730:2200::5474:8024
7 13 ms 16 ms 14 ms 2001:4860:1:1:0:1aae:0:19
8 14 ms 15 ms 20 ms 2a00:1450:8000:219::1:1
9 20 ms 15 ms 17 ms 2001:4860:0:1::5fa
10 19 ms 18 ms 16 ms 2001:4860:0:1::219d
11 16 ms 20 ms 12 ms ams17s01-in-x0e.1e100.net [2a00:1450:400e:80b::200e]
Trace complete.
Tracing route to www.twitch.com [54.231.49.193]
over a maximum of 30 hops:
1 4 ms 3 ms 2 ms 192.168.178.1
2 * * * Request timed out.
3 * * * Request timed out.
4 127 ms 129 ms * us-was03a-rd1-ae102-0.aorta.net [84.116.130.122]
5 122 ms 127 ms 118 ms us-was03a-ri1-ae11-0.aorta.net [84.116.130.165]
6 121 ms 123 ms 127 ms 213.46.182.202
7 135 ms 142 ms 120 ms 54.239.111.240
8 120 ms 140 ms 117 ms 54.239.108.197
9 156 ms 143 ms 149 ms 52.93.24.142
10 132 ms 119 ms 118 ms 205.251.244.67
11 * * * Request timed out.
12 * * * Request timed out.
13 * * * Request timed out.
14 * * * Request timed out.
15 * * * Request timed out.
16 * * * Request timed out.
17 * * * Request timed out.
18 * * * Request timed out.
19 * * * Request timed out.
20 127 ms 126 ms 119 ms s3-website-us-east-1.amazonaws.com [54.231.49.193]
Trace complete.
The traceroute shows the problem quite clearly.