If you push the flushing handle ones and the toilet flushes repeatedly 3 times it is most likely the flushing mechanism / tank valve problem. What exactly is slow on the first floor?
If it’s toilet drain than you might have a clog inside the toilet or in the drain line. Vent problem usually results in gurgling sound or sometimes it slows the beginning of flushing only. If it’s the tank that is filling up slowly it could be the tank valve, water supply line shutoff valve or water line itself (if its old, corroded, contaminated with mineral deposits, etc.)