Stiff Shifting Levers
The resistance is likely caused by friction in the cables/housing or something to do with routing. It's a bit suspicious that it's happening in both shifters though.

To track the source of the resistance down, start by detaching the derailleur, pull the cable put some tension on it and work the shifter to test for stiffness. Work towards the shifter taking the cable out of housing sections and retesting at each step.

If the shifters them are binding up under tension, you should be able to replicate that when you have the cable out of all sections of housing, emerging directly from the shifter.