If every large JS app on Safari is 10x slower than it is on Chrome, and the user base of Safari has been dealing with that since who-knows-when, and no developer noticed because they're all testing on Chrome, that's a problem.
Yeah. A problem Safari's developers should address. Apple is not a wee baby company that lacks the resources to keep up with Chrome in any dimension.
Or they are all compiling their JavaScript to legacy JavaScript specs for WebKit.
It's Apple's problem to solve though, I mean, outside of developers writing code that's more efficient to begin with so it doesn't really matter when it's 10x slower, but they should probably be doing that anyway.