Work through the causes in order: clean the pivot, then lubricate it, then check pivot tension, then look at the detent. Most gritty or sluggish action is dirt rather than a mechanical fault, and flushing the pivot with warm soapy water and drying it thoroughly fixes the majority of cases.
Pocket lint, dust and dried oil are the usual culprits. Flush the pivot with warm soapy water, work the blade to move the water through, rinse, then dry thoroughly with compressed air or by working the blade repeatedly on a towel.
Dry it completely. Water left in a pivot is how a smooth knife becomes a rusty one.
One drop of light oil each side, cycle, wipe the excess. See oiling a pivot.
If it is still tight, the pivot screw may simply be over-tightened. Back it off a small amount and check for blade play.
That is usually the detent rather than the pivot: a worn ball or a deepened dimple means the blade never gets the kick it needs. An over-tight pivot produces the same symptom, so rule that out first.
A genuinely worn detent is a manufacturer matter, not a home fix.