Netgear ex6200 extender very slow

There could be several things causing problems. You need to narrow it down to where the slow down is coming from and if its a user error or a hardware problem.

the EX6200 lets you select which band you connect to and if you want to use fastlane technology or not. If it was me, this is how i'd trouble shoot.

1. Hardwire into the extender. Speed test from there. If you're speeds are slow even hardwired in, its something in the backhaul. That could be: A. to far from router, if you're extender is to far from the router you're not going to have much speed. better to be a bit closer and have high throughput than to far and have slow speeds. B. using 2.4ghz to extend the network (lower bandwidth) C. wrong channel. you could be getting interference from other wireless channels. 

2. check your fastlane setup to make sure you have for your specific situation. With one setting it gives high throughput at the cost of sacrificing a band, the other will cut your bandwidth in half but still allow you both bands. Play with these settings to optimize how you'd like it.  read more here

https://kb.netgear.com/24662/What-is-FastLane-Technology-and-how-do-I-configure-it

try playing with these and let us know how it goes.