AssetBundles webGL slow post-download initialize

Hello! My project in Unity 5.6.3p2 (64-bit) (bundles builded, and project too). WebGL.

I have 300 ABs, every of 103kb in compressed size, LZ4.
604kb is incompressed size, but for this test mostly unusable.
Every AB contains single binary file.

I download 300 ABs from my server by code like this

Code (CSharp):

1. private const int AssetBundlesCount = 300;
2. private long _loadAllAssetBundlesTimeStart;
3. private int _assetBundlesLoadedCount;
4. readonly AssetBundle[] _loadedAssetBundles = new AssetBundle[AssetBundlesCount];
5.  
6. public void LoadAllAssetBundles()
7. {
8.     LoadAllABsButton.gameObject.SetActive(false);
9.     TextTiming.text = "loading..";
10.     _loadAllAssetBundlesTimeStart = DateTime.Now.Ticks;
11.     _assetBundlesLoadedCount = 0;
12.     for (int i = 0; i < AssetBundlesCount; i++)
13.     {
14.         string numStr = (i + 1).ToString();
15.         StartCoroutine(
16.             LoadAssetBundle(
17.                 GetAssetBundlesUrlByPlatform() + "/ab" + numStr,
18.                 5,
19.                 i)
20.         );
21.     }
22. }
23.  
24. private static string GetAssetBundlesUrlByPlatform()
25. {
26.     return "MY SERVER";
27. }
28.  
29. private IEnumerator LoadAssetBundle(string downloadPath, uint version, int index)
30. {
31.     while (!Caching.ready) yield return null;
32.  
33.     const uint crc = 0; // not used
34.     using (UnityWebRequest www = UnityWebRequest.GetAssetBundle(downloadPath, version, crc))
35.     {
36.         yield return www.Send();
37.  
38.         if (www.isError)
39.         {
40.             Debug.LogError("AB load error: " + www.error);
41.         }
42.         else
43.         {
44.             AssetBundle bundle = DownloadHandlerAssetBundle.GetContent(www);
45.             _loadedAssetBundles[index] = bundle;
46.             if (bundle == null) Debug.LogError("bundle == null");
47.  
48.             _assetBundlesLoadedCount++;
49.             //Debug.Log("AB loaded count = " + _assetBundlesLoadedCount);
50.             if (_assetBundlesLoadedCount == AssetBundlesCount)
51.             {
52.                 TextTiming.text = "timing1: " + new TimeSpan(DateTime.Now.Ticks - _loadAllAssetBundlesTimeStart).TotalMilliseconds;
53.                 Debug.Log("ALL ABs LOADED");
54.                 LoadConcreteButton.gameObject.SetActive(true);
55.             }
56.         }
57.     }
58. }

* LoadConcreteButton and some variables (AssetsCountForConcreteLoad, _loadedAssetBundles) is for NOT-INCLUDED-IN-THIS-TEST LoadAsset from loaded ABs. This test is only for measuring UnityWebRequest.GetAssetBundle and DownloadHandlerAssetBundle.

When I press my 1st button, which invoke LoadAllAssetBundles(), I monitor to Firefox and Chrome Network tab, I see what:
-- All 300 ABs are downloaded in 25-27 sec (iftop on webserver is confirms this)
-- Text TextTiming.text = "timing1: " .. is updated later
* after another 3-20 sec in chrome (for example 25 sec for download and 20 sec for magic stall)
* everytime in Firefox = after 60-100 sec

Some screenshots:
chrome 25 sec download, and 25 sec final measured - https://prnt.sc/hgu9f0
chrome 25 sec download, and 30 sec final measured - https://prnt.sc/hgu8pb
chrome 24 sec download, and 43 sec final measured - https://prnt.sc/hgu9xq
firefox 23 sec download, and 188 sec final measured - https://prnt.sc/hgu93b

Tested on browsers:
-- chrome portable 62.0.3202.75
-- firefox 57.0 latest

Tested on webservers for files:
-- Apache/2.4.29 (codeit) OpenSSL/1.1.0g: on http 1.1, https 1.1, https 2.0
-- nginx 1.13.7: on http 1.1, https 1.1, https 2.0

What is the work, on which a browser is stalled? I have monitored win 7 disk performance, and not see many reads/writes on disks. ABs is LZ4 compressed and after download, a browser don't decompress it.

**
also, why https 2.0 is mostly (80% tests) slower than https 1.1? Is it due to browser incorrect implementations or web-server implementations, or UnityWebRequest?

 

I have re-test this: it freeze due to developer console in browser when webgl block not in focus, and after click in center of screen - all resources finish downloading; and this "bug" doesn't exist when console is closed