Ferrari was fastest at the end of the fourth day's testing in Jerez, and finished in the top three on all four days, with Raikkonen posting the fastest overall lap in the SF15-T.
However, the former world champion warned that the lap times this early in the season are not the best indicator of how the cars will do when the Formula One season starts in earnest.
He said: "It doesn't mean much. The lap time can be anything but we've got a good amount of laps and things are working, we're not really having many issues. That's the main thing plus the feeling is pretty fine, so that's good.
"I think the whole package is better all-round; the engine and the car itself, everything has improved. Obviously we still have a lot of hard work to try and improve things but it's a good starting point. Compared to last year I found it much better but we still have a lot of things to work on and improve."