Installation :
Simply extract the folder of the time scale of your choice in your Phantom Pain folder (default Steam\steamapps\common\MGS_TPP).
For precaution, you can make a backup of 01.dat.

Know issues :
When using the cigar, the time scale return to the game's default (20), so better avoid to use it atm. In all case, it comes back
to your selected time scale everytime there is a loading (ex: returning to mother base/CCA, reloading a checkpoint, loading a cutscene, etc...).

For information :
The game's default time scale is set to x20.
The rain and storms last longer, depending on the time flow.

Enjoy.