1997 Mercury Marquis Pinging and long cranking time

For the past 6 months I have had a persistent pinging in the engine when it gets warm and I need to accelerate, such as going up a hill. Have tried new Egr valve, new O2 sensors and changed cats. What else can I do. Oh have also replaced fuel filter. Last 3 days it is taking longer for engine to fire up when starting it, has gone from almost instant start to at least a 45 second crank time. What should I check for this. Ty so much for your time and answer.