a TBM won't care if it has to dig the whole
tunnel itself or theres a small tunnel already there , it will take more or
less the same time.
It won't you know. If it doesn't have to dig the middle bit, it
doesn't have to do the work. That means less work needs to be done.
And that means it takes less power. And that means that more power is
available to put into the work it does do. And that means its more
efficient, and it finds that work easier

Plus why inflict a windy route on a new
rail line when for high speed it needs to be as straight as possible.
If its going to have to stop every few hundred yards for a station, it
hardly matters whether high speed is possible or not. Its not that
windy anyway.