I recently put on a new set of STI shifter/brake levers (Dura Ace 9- Double). I'm generally very happy with them; very smooth. The right lever works great. The left lever brakes fine and upshifts from small chainring to large just fine. However, downshifting from large chainring to smaller feels stiff, hard. Anyone know if there's an internal adjustment for this, or some way to get it so it's not so stiff? Thanks!!

Is it sluggish shifting into the small ring, or is it hard to actuate the lever? If it is slow to shift, your cables/housing may be dirty, crushed, or kinked. Start by checking those. If the shifter itself is stiff, me not know...

Cables and housings are all new and smooth. Once the shifter clicks, the shift is just fine. It's the click that is stiff and hard.

Sounds like it needs some miles...