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

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.

Asked by for the 1996 Hyundai Accent