Check simple stuff first. Sometimes the inside rubber flange of the OE handgrips rub against the throttle housing and this causes drag. You may be able to warm the grip and massage/puull it outward just a tiny bit to get that area to clear. I little spritz of silicone lube between the rubber thand the housing further sliks it. If you have added risers, binding or over stretching of ths cables is a reasonable possibility. My tunnel filters but mine in a bind once. If the cables themselves drag, I would suspect a frayed cable that needs replaced, although I would be tempted to lube it as the book says.
prs