Per https://images.anandtech.com/doci/13392/SPEC2006-eff_575px.p...
SPECint2006 -- 36.93 --> 44.92 is about 20% better
Speedometer2 -- 90 --> 125 is 38% better
That is nearly 2x what SPECint would predict. (It's also not changes in Mobile Safari / iOS 12 because every device benchmarked was on the same version of iOS 12).