VDiff finds the current position of the source master and then waits for the target replication to reach
that position for _filtered_replication_wait_time_. If the target is much behind the source or if there is
a high write qps on the source then this time will need to be increased.
You can follow the progress of the command by tailing the vtctld logs
VDiff can take very long (hours/days) for huge tables, so this needs to be taken into account. If VDiff
takes more than an hour and you use vtctlclient then it will hit the grpc/http default timeout of 1 hour.
In that case you can use vtctl (the bundled vctlclient + vtctld) instead.
There is no throttling, so you might see an increased lag in the replica used as the source.
VReplication and VDiff performance improvements as well as freno-style throttling support are on the roadmap!