Steps to reproduce:

1) Download attached project 'WWWLags.zip' and open in Unity
2) Install XAMPP
Download XAMPP from here: https://www.apachefriends.org/index.html
3) Download attached PHP server(attached in Edit field)
4) Create folder and rename it to 'Bug' in C:\xampp\htdocs
5) Add PHP server('index.php') and 'Json.txt' files in recently created 'Bug'' folder
6) Open 'index.php' file with text editing program('Notepad++') and add 'header("Access-Control-Allow-Origin: *");'
7) Launch XAMPP's Control Panel
8) Start Apache module
9) Open browser and enter : http://localhost/Bug/index.php
10) Focus back on Unity and open 'WWWLagReproducer' script
11) Change Url for PHP server to 'http://localhost/Bug/index.php'
12) Switch platform to WebGL and build 'Main' scene
13) Launch project on the browser and click on 'Request' button
Note that browser will freeze (Not Responding) for ~ 1 to 2 minutes

Expected result: Getting requested json string should not freeze the browser
Actual result: Browser freezes when getting requested json string

Reproduced with: 5.6.0b1, 5.6.0f1, 5.6.2p4, 2017.1.0p1, 2017.2.0b4, 2017.3.0a1

Not reproduced with: 5.5.4p2, 5.6.0a1

Regression since: Between 5.6.0a2 and 5.6.0a5 (project crashes when trying to open with those versions of Unity)

Browser under testing:
Firefox 54.0.1