perf: improve ssr performance (#1068)

* perf: improve ssr performance and resolve memory leak

* reduce to normalized pages map

* assign and return map in reducer function

* return complete map from reduder function

packages/@vuepress/theme-default/util/index.js

@@ -58,13 +58,11 @@ export function resolvePage (pages, rawPath, base) {
     rawPath = resolvePath(rawPath, base)
   }
   const path = normalize(rawPath)
-  for (let i = 0; i < pages.length; i++) {
-    if (normalize(pages[i].regularPath) === path) {
-      return Object.assign({}, pages[i], {
-        type: 'page',
-        path: ensureExt(pages[i].path)
-      })
-    }
+  if (path in pages) {
+    return Object.assign({}, pages[path], {
+      type: 'page',
+      path: ensureExt(pages[path].path)
+    })
   }
   console.error(`[vuepress] No matching page found for sidebar item "${rawPath}"`)
   return {}
@@ -128,12 +126,16 @@ export function resolveSidebarItems (page, regularPath, site, localePath) {
   }
 
   const sidebarConfig = localeConfig.sidebar || themeConfig.sidebar
+  const normalizedPagesMap = pages.reduce((map, page) => {
+    map[normalize(page.regularPath)] = page
+    return map
+  }, {})
   if (!sidebarConfig) {
     return []
   } else {
     const { base, config } = resolveMatchingConfig(regularPath, sidebarConfig)
     return config
-      ? config.map(item => resolveItem(item, pages, base))
+      ? config.map(item => resolveItem(item, normalizedPagesMap, base))
       : []
   }
 }