It could be a lousy toilet! Do you know what brand and model it is?
There could be an obstruction in the trap. There could be a restriction in the drain line. The tank fill level may not be right. The bowl fill hose may not be pointing down the overflow, and the bowl doesn't have the proper level. There could be deposits in the rim jets or the siphon jet (assumig it has one, not all but most do) restricting the rate at which the water is released. You might have a generic flapper valve that is not releasing the proper amount of water. And the list goes on.
A description of the actual flush operation and draining would help.
Did it ever work different, or has it changed recently?