Spend the money and get some Gore cables.
Then use graphite in the shifter itself.

(edit) Seems Gore is no longer in the cable business. Find a type of cable that has its own inner tube for the actual wire to run through. That made a world of difference in mine.

Aside from the obvious of making sure there are no sharp bends in the housing, try a premium cable set. I've had good luck with JAGWIRE cables...

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Remember Luddites are people too...

Is it possible that you have the lever fixing bolts over-torqued? Are the levers hard to move with the cables detached or slackened?

http://www.shimano.com/...6577569830605258.pdf

My first guess is that you need to review the cable routing. You have a pinch, tight radius, or the cable is rubbing as it enters/exits the housing or the internal frame routing. On some older frames -- poorly designed enough to not fit/mate with the outer diameter of the cable housing -- I've made small wedges to force the cable to the correct angle for entry into the frame.

If you have any, remove any tape (even bar tape) being used as a "strain relief" for the housing; frequently the culprit. Then, test the shifting as you add any back. If that doesn't do it, disconnect the cable from the derailleur and work your way along the route to find where the restriction is.

Since your bike is a 2008 and it's now 2015, I assume you've replaced the cables a few times, right? If not, time to do it now. And just maybe the original installer accidentally bent the cable somewhere in the middle and continued to install it anyway...bad practice, but it does happen.

2015 microSHIFT BS-C11 Carbon Lever TT Bar end Shifter

MicroShift came on my Felt originally. I replace with Dura-Ace last year, but I like the MicroShift ones better. Much lighter action than the Dura-Ace.

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Typical ST solution. Buy this. Buy that.

Try troubleshooting the root cause first.

As suggested, first narrow down the problem. I would disconnect the derailers from the cables and then pull the cables out just a bit so they shifters don't need to move them. Then see if the shifters are easy to move. If they are much easier to move, then fix the cables, if not then fix the shifters.

I know in my case the left shifter always seems like it is more difficult to move than I like while the right index shifter is pretty easy.

If it's not the cables, remove the bolt, open up shifter and using a q-tip shear a little, I repeat a little, white grease on he surfaces. It works wonders.

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''Sweeney - you can both crush your AG *and* cruise in dead last!! 😂 '' Murphy's Law