1996 Hyundai Accent Question: my car doesn't have power,very slow, jerking when driving. what could be wrong

this car doesn't have speed it doesn't have power the engine makes noise when idling and it takes a lot of fuel. i recently changed the oil, plugs, air filter, oil filter and petrol filter and it didn't solve the problem. i took the cylinder head for scheming, changed rings and gaskerts still no change. it's a fuel injected engine.