Increase download parallelism.

src/sidebar.js

@@ -53,7 +53,7 @@ class Book{
         return fetch(url, {
             credentials: 'include'
         }).then((res) => {
-            console.log(`Downloaded.`)
+            // console.log(`Downloaded.`)
             return res;
         }, onError)
     }
@@ -118,7 +118,7 @@ class Book{
                         .then((json) => {
                              return this.extractChapterAssets(json)
                         })
-            },{concurrency: 1}))
+            },{concurrency: 10}))
 
         return Promise.all(downloads)
     }
@@ -152,13 +152,12 @@ class Book{
             return this.downloadResource(url)
                     .then((res) => {
                         if (res.ok){
-                            console.log(res.headers.get('Content-Type'))
                             this.book_files[url].headers = res.headers
                             this.book_files[url].mime = res.headers.get('Content-Type')
                             this.book_files[url].body = res.blob()
                         }
             })
-        },{concurrency: 1})
+        },{concurrency: 10})
     }
 
     insertBookFile(url, filename){