Compare the benchmarks with 2012 imac and mba. Then you will have an idea how its gonna happen.
I've just looked at the benchmarks, the new 2018 Macbook Air just about beats my 2012 iMac in single core. Multicore the iMac beats it by around 5,400 points.
It's interesting tho that the iMac can be beaten even if its only in single core, by the new Macbook Air. Yet it seems the iMac is perfectly fine for what i do. It runs Pixelmator Pro fine, i can write and edit photos and videos in iMove and even Final Cut Pro X. But then i did get the iMac with the i7, 16GB Ram and the 3TB Fusion drive back in 2012.