Test it: run the bath and the basin. If they drain normally, the restriction is local to the WC. If they are also slow, the blockage is in the common run and needs clearing from a chamber, not from the pan.
Partial blockages usually mean wipes, sanitary products or a build-up of limescale in a hard-water area gradually narrowing the pan outlet.
Left alone, a partial blockage becomes a full one, usually at the least convenient moment. Clear it while the water is still moving.