If it were the cable adjustment, you'd see the same sluggishness on most or all the upshifts. The fact that it's limited to the last shift argues that it's either the limit adjustment, or the cable hanging up a bit.

You can test as I suggested earlier and confirm that it shifts well without the cable, or you can simply back the limit off a half turn and see if it makes a difference. If yes, then bring it back to where it was, and back off by increments until the problem is solved. OTOH - if moving the limit out doesn't change anything, you're back to cable friction.