Best moment for time stretching?

I have a project with acoustic guitar, double bass, midi piano and vocals. Perfect, but the tempo is too slow. It has to be speeded up 10 to 15 bpm. My question is what is the best moment to time stretch:

  1. Before mixdown, increase the song tempo and apply time stretching to all audio tracks individually
  2. After mixdown, apply time stretching to the stereo master

My intuition says before mixdown: time stretching is applied to less complex audio material and the sound of the midi piano is unaffected.

In my opinion the mixdown should should look like the project. So do the time streching procedure before the mixdown.
You can use your mixdown to speed up the tempo to check if this tempo fits, but in the end i would speed up every single track.

Btw. the algorithm works better with less complex material, so before mixdown is best practice.

Since phase relationship is not being kept when stretching / warping multiple tracks, it depends on the project. If you have many tracks that rely on Phase relation (multimiced signals) stretching the final mix or stems will yield better results than stretching single tracks

Thanks for the swift replies. Only the guitar is multi-miced (contact mic and room mic). Would it help if I made a submix of the guitar to one stereo track? I presume the phase relation between de right and left tracks of a stereo track are left intact after stretching?
Otherwise, just try both options and seeā€¦ (sorry, hear)