There is some ambiguity about enabling and disabling IPv6. Even 6 years ago where things were much worse for IPv6, Cisco claims that it shouldn't be disabled.
Some people connecting to dual-stacked sites notice connection delays that will disappear once IPv6 is disabled. This can only happen if the IPv6 path has some kind of problem which causes the IPv6 connection to time out before falling back to IPv4.
Later on, the writer says that
Sometimes, connections fail completely until IPv6 is disabled. Misconfigured or buggy DNS resolvers may cause dual-stack connections to fail by erroneously returning an NXDOMAIN reply to a AAAA request. Misplaced IPv6 routers can inject information into the network which draws packets into black holes, wreaking as much havoc as a rogue DHCP server. Look for unauthorized devices sending IPv6 Router Advertisements, or enable first hop security features.
What Should You Do?
If you notice that IPv6 needs fixing, fix it today rather than postponing until tomorrow. Many popular troubleshooting regimens simply prescribe disabling IPv6 as the “solution,” which really does nothing more than to hide the underlying problem with the IPv6 network. When you have a network problem that is “solved” by disabling IPv6, you have masked the symptom of a bigger problem that warrants further investigation.
However as @RonMaupin said, this was fixed later on by Happy Eyeballs.
At a different forum they say that:
From Microsoft's perspective, IPv6 is a mandatory part of the Windows operating system and it is enabled and included in standard Windows service and application testing during the operating system development process. Because Windows was designed specifically with IPv6 present, Microsoft does not perform any testing to determine the effects of disabling IPv6. If IPv6 is disabled on Windows 7, Windows Vista, Windows Server 2008 R2, or Windows Server 2008, or later versions, some components will not function. Moreover, applications that you might not think are using IPv6—such as Remote Assistance, HomeGroup, DirectAccess, and Windows Mail—could be.
And to top it with this
Problems With Disabling IPv6
Disabling IPv6 can cause problems. If your Internet connection and router have already migrated to IPv6, you’ll lose the ability to use it properly. IPv6 may also be required for some home networking functions — for example, the easy-to-use Homegroup home networking feature introduced in Windows 7 requires IPv6 enabled on the computers on your home network to use it.
The entire world is moving towards IPv6, although it’s happening too slowly. IPv6 is necessary to replace IPv4 — we’re running out of IPv4 addresses and IPv6 is the solution.
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There’s a good chance you don’t actually need IPv6 on your network — unless you rely on Windows Homegroup or similar features — so it may not be particularly harmful to remove if if you know what you’re doing. However, you won’t see a speed improvement from clinging to IPv4 unless there are serious problems with your Internet service provider’s network or your home network.
Also have a look to this article for more information about IPv6:
What Is IPv6, and Why Does It Matter?
IPv6 has come a long way the past years. If you are trying to speed things up, you might not see any difference. It does not only depend if your ISP has IPv6. Websites use IPv6, your PC uses it in different ways, servers you connect to use IPv6. So the best way to know? Test it and check if things get faster, slow down or about the same. If you are asking me, try it yourself. If you see an improvement keep it disabled. If not, re-enable it. But be prepared to check for the aforementioned problems or raised red flags after disabling it.
