This is a side-effect of the fix for MDEV-8083 MTR is broken on systems with IPv6 disabled https://github.com/MariaDB/server/commit/a453a280403faef3377db0441560c57b4ae1235b
The fix makes server to listen only on IPv4 by default, rather than IPv4 + IPv6

the client tries to connect to addresses returned by getaddrinfo() , in the order in which this function returns IP addresses. So, getaddrinfo gives 2 addresses for localhost, the IPv6 and IPv4, in this order. Prior to MDEV-8083, server was listening on both IPv4 and IPv6, this the very first attempt to connect would succeed. After the MDEV-8083 fix, attempt to connect to IPv6 address fails, and the failing attempt takes some time (long time to connect to non-existing address this might be Windows specific)

So, what could the fix be? Can we just tell the client to connect to IPv4 address, like --host=127.0.0.1 ?

There would improve the speed, in general case.

For example we could change fix_host() in lib/My/ConfigFactory.pm to return "127.0.0.1" rather than "localhost" . Connections that are done within tests itself, ánd reference localhost, like

connect(con1,localhost,root,,);

would still be somewhat slower. Maybe this is not a big deal.

Maybe adding --bind-address=* to the server options on Windows would help as well.

On the other hand, perhaps it is possible to fix mtr to ignore the warnings from MDEV-8083 (since those are not really errors), and let bind-address as default ?

Ignoring warnings from MDEV-8083 is kind of fine, but we need not to ignore them in ipv6 specific tests. Currently there's no way to enable a warning suppression globally and undo it for specific tests. But, of course, we can add it.

Specifying --bind-address=* on Windows, again, would be fine, but at the moment my.cnf files don't have per-os sections and mtr my.cnf templates are not per-os either. Still, we can have fix_bind() function that will return * for Windows and 127.0.0.1 otherwise. That should be easy to do.