Huh?
Since KitKat I think, WebView is updatable (and is on the App Store), plus browsers like Chrome and Firefox can provide their own WebView implementation. Most people with non-ancient phones are probably using Chrome's engine in all WebViews.
And also, old engine doesn't even mean "slow".
Only Apple used to an actual problem here, because for a long long time there was no JS JIT in WebViews.
I have tested several demos by ionic, framework7, etc... is amazing how slow are androids...
Apple hardware benchmarks well beats Android hardware: http://browser.geekbench.com/mobile-benchmarks
Given Safari can optimise for a single GPU target (which is also probably faster), it is likely the speed differences are not due to the browsers per se.
I've been dealing with UIWebView and WKWebView problems for over 3 years now, but Android webviews have given me zero problems.
You mean Google. Android webviews are VERY slow. Tha comnpound with the overall bad android OS (terrible privacy and security, terrible overal performance).