Your WA850RE is running in latest firmware?
When browsing webpages, is it working properly?
Did you connect directly to your main router to test?
How many signal lights on WA850RE?

I've looked firmware updates, but not in that part gets the latest update and the version which is, I read that may be damaged if I put a bad upgrade or something, explain it to me please.
If it works well, I whole load 1080p videos in seconds.
I've tested without the Range Extender and I get only 2 lines of Internet and web pages are slow to load as the videos.
When I plug the Range Extender usually have between 4 and 5 lines.

All that tells me is this firmware:
Firmware Version: 3.14.8 Build 130922 Rel.53068n
Hardware Version: v1.2 00000000 WA850RE
This is good? Old? Might get old if the new version please?

The firmware is old. Download the latest version from the link as below.
http://www.tp-link.com/en/download/TL-WA850RE_V1.html#Firmware

Range extender cannot help to improve internet speed while the speed will be decreased normally because of Half duplex Communication.

I tried it with the updated firmware, and keeps going the same problem.

I've tested without the Range Extender... web pages are slow to load as the videos.

Maybe you should contact your ISP to improve your internet speed. As I said, RE can help to improve Wi-Fi signal but cannot improve internet speed.