...and the sear engagement...and the friction from contacting the frame...and the pivot pin.
Joe511,
While I applaud your idea, it's just not as simple as "fixing" one part. If it were, S&W would be doing it to sell more guns.
Now, change that to, "What part has the most influence on trigger pull weight" and we can start addressing it. There are definitely some parts that have more affect than others.
The first, and most obvious, is sear engagement. Polishing the sear and striker tang will help. Reshaping the sear will help a lot more, but is very risky work. Screw up the sear and you'll be wishing you had called Apex at the first.