Some people think an old toilet from the 1950’s has seen its last days. Sometimes the water will spin around and almost overflow before it flushes. I tried to clean the jets around the bowl with a coat hanger but without any better results.
Whenever diagnosing a questionable toilet problem give it the bucket test. The bucket test uses several gallons of water poured into the toilet bowl. Pour it as fast as the toilet can take it. Did the water go down lickety-split? If it did, the toilet is not clogged internally, nor is the drain line. Do it again, and now listen to the shower and sink drains. Did either of them gurgle? If they did not make a sound, it indicates the vent to the roof is not plugged.
Why is the toilet not flushing correctly? My guess it is probably calcium buildup in the jets in the upper rim of the toilet bowl. This is very common if it is about 30 to 40 years old. In the old days, my father sold muriatic acid to clean the jets. He had you pour a quart of it into the overflow tube in the toilet tank, wait 20 minutes and then flush. Sometimes this procedure could take several doses to completely clean the jets in the rim.
Today we have a toilet delimer by ROOTO that does the job better and quicker. We add a step by having customers remove about one half of the water in the bowl, not the tank, before pouring the delimer in the overflow tube. This enables the product to remove some of the buildup in the trap downstream of the bowl, (a place most of you will never see). It has also been known to clean stains at the same time.