Hey Zaibakk,
If you create a graphic intensive game for HTML5 it’s going to take a while for people to load up on their computers. The reason being is because it’s being downloaded by their service provider each time they go to your website. 32-bit browsers, just like systems have a limited amount of memory. If you’re trying to make an intensive game, you may want to focus more on a 64-bit browser, or even a platform game (such as Windows or Android/iOS).
Please also keep in mind that not all browsers support HTML5 and they don’t all support HTML5 at the same level. Currently, Chrome is having some issues with HTML5, and I’ve seen some issues with FireFox as well. However, seems to run HTML5 projects without any trouble. However, if it’s a large content game, it takes a while to load.
If you wish to know whether or not a game will run on a browser on a cellphone, you’ll need to look at the cellphone operating system and see what is supported.
The Unreal Engine is always making improvements and corrections to any bugs found within the engine. I know that a few HTML5 related issues are being fixed during 4.8 and will continue to be added to. What I would suggest is for you to read over our forums to gather additional information. Especially take a look at our section where our [release notes][1] are listed.
Have a great day! 
https://forums.unrealengine.com/forumdisplay.php?8-Announcements-and-Releases