Indeed, a 10%-20% difference is so much that airliners would sell their grandmother (if they had one) for a leap like that. A 2% improvement is considered significant and is a massive economic incentive, even more so these days for long-haul flights which are now in some cases 50% longer due to conflicts, and at times extreme price hikes for jet fuel.

(This applies a bit less to military engines though, where performance is a more important factor. Fuel efficiency is still important though)