After years of using, the toilet is getting harder to flush even the water level is not low. If such a case, most probably is the siphon unit problem where the diaphragm worn out.
To take the siphon out, we need to open up the toilet tank using wrench. Once take out the siphon, we can see that one side of the diaphragm is broken.
U can buy the diaphragm at local hardware shop, around RM1-2 /pcs only. Or u can use any hard cover, just cut accordingly.
Once replaced, put back everything and install back the siphon. Make sure test the diaphragm, when we pull it should move up and bounce back due to the spring.
Once we installed back everything, we can test flush the toilet and it should flush easily.
Below is the structure of the toilet. Normally the most we need to replace is the handle, inlet valve, and siphon diaphragm. We can save a lots by doing it yourself instead of calling plumber.