>...each of this sub-folders have 25 thousand files and around 130 Gb, and we do not
>notice any slow down... except when we run 7Zip for the folders that are more than a
>year old... that takes around 36 hours to run!


We did some testing recently on 7-Zip on a directory tree (main, plus two sub-directories) of mostly DBF, FPT, and CDX files.

Using these levels for compression, 303 files, 375 MB, compressed down to:
1 = 4s, 19.9MB
3 = 4s, 19.9MB
5 = 59s, 16.4MB
7 = 82s, 15.5MB
9 = 180s, 14.8MB

If storage space is not the primary concern, switching to a compression of 3 yields 15x to 45x better performance for at most a drop of 25% in storage space. It might even be worth compressing using 3, then to recompress the smaller file using a higher value.