Why does Rapid Fire mod make my gun shoot slower?

There are 2 factors that can cause the issue:

1. Rapid Fire SPS exceed the speed cap. In most games, your gun will either shoot slower or won’t change fire rate at all if you go above the speed cap pre-set by the game developers. To fix the issue, please change Rapid Fire sub-mode to the one with lower SPS.
2. Used with the incompatible weapon. When Rapid Fire is used with fully automatic weapons, it will make them shoot slower as their fire rate can’t be changed. For more info visit Instructions page, select your console type, go to All Mods and click on Rapid Fire icon.