Despite being such an awesome mod, the 300% mining speed of argonians was really bugging me. It just always felt like cheating. So, after some researching I've come up with the solution.

Go to: "Steam\steamapps\workshop\content\294100\959780399\Defs\ThingDefs_Races". There you'll find a file, called "Race_XArgonian.xml". Open it with any text editor and find a line, that looks something like this: <MiningSpeed>3</MiningSpeed>
And change it to this: <MiningSpeed>1</MiningSpeed>
There are also a few lines that alter speed of other actions, such as 200% research speed and 255% stonecutting speed. Change them to "1" too if you want the default speed multipliers.
After all your meddling with the values save the file and start the game. Your problem should be fixed.

Oh, and a huge thanks to the mod author, Xen. Sorry for messing with your work, it's still splendid!