Mostly at slowing down iOS runtime performance per clock cycle, and increasing compilation times.
Its also been hugely successful at punishing companies for using it, since it didnt really work for a few versions.
While there have been fewer crashes (sort of) due to the switch, as someone who worked for years in Objective C and years in Swift, I honestly never felt that Swift significantly improved much except allocation rituals and properties.
I do however enjoy Kotlin for the backend because it has some nicer semantics for object construction.
While I do admire the work that Apple has put into Swift, I really felt that Apple was trying to "impress everyone", with a whole lot of bikeshedding and sick amoutns of code generation to convince us it was the right thing to do.
Frankly, apps run over 10 times faster in obj C and xcode with Swift is probably my least favorite and sort of non-ergonomic experience close to DOS batch file dev in Windows Notepad.