[corty]

When a server has IPv4 and IPv6 connectivity, i.e. an A and AAAA DNS record, clients would usually try IPv6 first, then IPv4. Now, if IPv6 is broken somehow (as it was prone to be due to tunneling, Teredo and even nowadays stupid ISPs), the client would wait for a connection timeout, then try IPv4, so every connection would hang for one timeout.

Nowadays, most clients, especially browsers, will try IPv4 and IPv6 concurrently and use the connection that comes back with an ack first. This prevents hangs at the expense of a little higher server resource usage.

But the myth of "IPv6 makes things hang randomly" remains.

[formerly_proven]

It's not a myth when it happens. Last time I tried v6 again on my network (about a year ago) I had these issues. Not a lot of them, but toggling v6 off made them go away. There are fewer issues, true, but having dual-stack does still appear to break some things. I say "appear to" because I'm assuming that it's not actually an issue with v4+v6 dual stack, but rather a problem with misconfigured services (e.g. services having wrong AAAA records, stuff using v6 that doesn't work properly with v6 etc.); from an end-user perspective this does not really matter: There are sometimes issues when v6 is on, which go away by turning v6 off - end user perception: v6 is wonky.

[mindcrime]

Your lived experience is what it is, and I certainly don't mean to contradict you. But just to add another viewpoint for perspective...

My ISP (formerly Time Warner, now Spectrum) has been handing out v6 addresses here, and I've been running in dual stack mode, for a good 3 years or more now. And in all that time, I can remember exactly one occasion where IPv6 caused any issue. And that was only because somebody at my employer who was setting up a GCP environment didn't take into account that anybody would be coming in on IPv6, didn't configure the corresponding firewall rules, and it broke using kubectl to access GKS clusters from my home network, as kubectl was defaulting to the v6 connection.

So yeah, v6 can cause some weirdness, even now (the event described above was about 3 months ago) but in my experience it is exceedingly rare. YMMV, of course.

[easrng]

I use Spectrum and their IPv6 connectivity drops out relatively frequently, but IPv4 connectivity stays.