Why is Drupal with redis slower than mariadb?
This topic should probably be closed, as the Drupal Redis module maintainer, I am taking over the issue in the module official issue tracker, specific questions are pending there, and a few micro-optimizations have already been done since this question was opened.

For everyone that would help us or need support, please consider participating to https://www.drupal.org/node/2550275 - this would be great help.

Thanks in advance to everyone that will give us a few minutes to solve this and improve the module.