fix: distracting and time wasting after importing smoothing scroll

Lervarage 'speedAsDuration: true' to ensure each transition only takes 400ms (Extracted from vuejs.org https://github.com/vuejs/vuejs.org/blob/master/themes/vue/source/js/common.js#L367)

Author: ulivz

packages/@vuepress/core/lib/app/root-mixins/updateLoadingState.js

@@ -6,7 +6,8 @@ export default {
       this.$vuepress.$set('contentMounted', true)
 
       this.$smoothScroll = new SmoothScroll('a[href*="#"]', {
-        speed: 1000,
+        speed: 400,
+        speedAsDuration: true,
         easing: 'easeInOutCubic'
       })