Has the performance changed? Did it do it all at once, or gradually?
Are you using a generic flapper valve or an original? Does it get better if you hold the handle down to use more water? What about the fill valve? Has that been changed? If you take a bucket of water and slowly pour it into the bowl, does the bowl level end up higher?
Do your showers and baths drain as fast as they did? Any strange gurgles or other noises from any drain?
If you have really hard water, the holes under the rim can become somewhat plugged. Check with a mirror. A wire from a coat hanger can often clean them out if plugged, but it could be any one of many other possibilities not related to the toilets themselves.