Changed my spark plugs for the 1st time @ 198k, didn't notice a big change even though the old plugs were clearly worn out by looking at them. Over the past month or so performance has seemed to improved, does the PCM take some time to adjust to the new plugs or should the effect be noticed immediately?

 

If they weren't misfiring, you won't see much, if any, difference.

 

If they were misfiring I didn't feel it and it didn't throw a code.