Really? In all the benchmarks I've seen, the latest iPhones handily beat the latest Android phones. (And old iPhones remain usable longer than old Android phones.)
Assuming the software doesn't get in the way, benchmarks are all about hardware, not software.
No, if the OS significantly slows down a benchmark, it's a bad OS. Both Android and iOS are fine in this respect. (It's a bit fuzzier for graphics tests, I admit, where driver quality comes into play.)
From what I understand iOS is pretty picky about background processes. Limit what can run in the background (and for how long) with android and you will probably get similar results.
A quick google search gives me results like this: https://www.tomsguide.com/us/galaxy-s9-benchmarks,review-519... "Galaxy S9 Is Fastest Android Ever, But iPhone X Is Faster"