If a network and SQL Server is not correctly configured to use IPv6 there can be speed related issues while the IPv6 settings are found not to be available and the IPv4 settings are used.  Microsoft have implemented the IPv6 before IPv4 sequence for many years.

Rather than disable IPv6 it is preferable to set Windows to prefer IPv4 over IPv6. This can be done at a computer level or as a Group Policy

Key:  Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters

See information in the following link:

https://support.microsoft.com/en-us/help/929852/guidance-for-configuring-ipv6-in-windows-for-advanced-users