Performance on Android 5.0+ with the webview is generally fine, it's normally the same as the Chrome browser since they share the same browser engine. In fact it's the same browser engine as Crosswalk, but more up-to-date. (Almost everybody who complains turns out to have a GPU-blacklisted device, which affects just a few % of devices.)
Eventually Android 4.x will shrink to a negligible amount of market share. You can keep using the last version of Crosswalk up until then.