Benchmarks I've seen indicate the i5 in the 2019 27" iMac is at least on par and perhaps a bit faster than the i7 in the 2017. It's 6 actual cores vs a 4 core processor with multi threading. I never really heard anyone say they felt the 2017 i7 was too slow. It's one of the things that convinced me to go i5 rather than i9 in the 2019 iMac. Doubt there's much, if any, real world performance difference between the 2017 i7 and current i5 iMacs. As Bohemiem noted, where you'd notice much difference between the i9 and i5 in the 2019 iMac would be when using software that really exploits the extra cores and multi-threading, or when running many programs simultaneously.