Open Radar

 
A full round JavaScript evaluation round trip in WKWebView, as measured by:

// start
[_wkWebView evaluateJavaScript:@"..." completionHandler:^(id result, NSError *error) {
   // end
}];

Seems to take ~5 ms on an iPhone 5c running iOS 8 beta 5. The equivalent UIWebView method (stringByEvaluatingJavaScriptFromString:) is about 10x faster (measured via the harness at https://github.com/mihaip/web-experiments/tree/master/uiwebview-communication).

Based on an Instruments profile (see https://www.dropbox.com/s/20vuhdv2siweymr/evaluateJavaScript.png), a large part of the time is spent in JSContext construction when parsing the return value. If caching of JSContexts is not feasible, then an alternate method where the return value is unparsed (or just always stringified) would be most welcome.

Relatedly, postMessage suffers from the same performance problem (see https://www.dropbox.com/s/d7djzqe3h075cs3/postMessage.png); it also spends a large amount of time in JSContext construction.