It might be more informative to say how much improvement in efficiency it achieves relative to RR’s current Trent engine. The first-generation Trent (the Trent 700) came out nearly 30 years ago.
However, the two most recent Trents are the Trent XWB and the Trent 700. According to Wikipedia, the Trent 7000 has a 10% lower SFC than the Trent 700; and the Trent XWB has an 11% lower SFC than the Trent 700.
So, a 25% improvement over the Trent 700 would be around a 14% improvement over the current engines, and no small achievement.