Assuming that your fixtures downstream of this toilet are draining properly and thus there isn't some kind of systemic backup, I would try flushing it a few times by holding the handle down to drain the whole tank and see how it performs. Sometimes, the lazy flush occurs because it isn't starting with enough water. If that doesn't work, I would hit it with a plunger. Good rapid vigorous strokes. Could be there's a partial clog somewhere in there. If the plunger doesn't work, then a closet auger. If the problem doesn't resolve itself with one of these methods, report that back and see what The Gang thinks.
What I'm recommending isn't necessarily the optimal solution, it's just basic Chicken Soup...can't hurt.
One other thought is: does the toilet seem to be dropping as much water as usual through the flush valve? I get a lazy flush occasionally off what I call a "misfire". This occurs when for whatever reason the flapper doesn't cycle all the way open and shuts faster than usual. Usually occurs if I don't push the handle all the way down before releasing. The bowl gets a little water in it, the tank starts refilling, but there isn't enough water through the valve to do a regular flush, just a slow siphon to the bottom.