I run (ran) the in-line brake levers on my cross bike for a season. They never caused any additional friction that I could detect. Braking was alway relatively smooth- until the races got all wet and goopy.
Nice when noodling about on the road, running to the store, etc. But since I've switched to a Midge drop bar... the in-line levers have been obviated.
Like Moose I suspect poor cable routing or some other problem with the cables and housing. The adjusting barrels of the in-line levers might be rough inside, or they might be installed improperly, forcing the cable to make a sharp bend.
One likely suspect, if cables are routed properly is that the cable housings were not cut and finished cleanly. When a cable hosing is cut, the end needs to be "finished" to ensure that the metal windings haven't compressed into the path of the inner wire. Sometimes, the blades of the cutting tool strikes in such a way that the metal winding will actually obstruct the inner wire or drag on it. One just has to snip off another piece. The plastic lining must also be properly flared out (like a funnel) at the opening. Typically, after cutting housing all one needs to do is use a metal punch (or nail or similarly shaped implement) to re-open the inner plastic liner and ensure that non of the metal windings are obstructing the inner wire.
All this is hidden by the ferrule, however and not visible until the ferrules are removed. So the shop needs to do more than lube the cables-- they're just being lazy.
Furthermore, you don't really need to lube cables anymore. The plastic housing liners are designed to reduce friction and most inner wires are "slick" (meaning that they've been extruded so their surface is smooth). Teflon coated inner wires (black in appearance) are even "slicker"!
Lubing cables can actually attract contaminants like dust and dirt and make things worse. If ANY lube is needed just a teensy bit of light-weight lube (many chain lubes like ATB or Pro-Link work double duty in this respect) is all you need.
Bottom line? Something is wrong about the way the bike is set up. It's not the in-line brake levers.