Slow wifi extender. No, REALLY slow extender.

I am posting this because I can't comment on any similar threads for some reason :P I don't know the precise model because it doesn't say what it is on the visible casing.

This device was easy to set up and use. However, it is on 24/7, and over time the speed deteriorates. I measured the download speed as 0.4Mbps via the extender and 75Mbps using a direct wireless connection to my BT router from the same PC.

According to similar threads, it is "typical" for an extender to cut the bandwidth in half. Half of 75Mbps would be fine, but 0.4?! 

Modern routers are supposed to negotiate a favourable channel for any connections, so how can I end up with 0.4Mbps when it is approx. 3m from the router? Perhaps the extender is "relegated" and the channel occupancy in my urban street is itself constrained.

How can I resolve this?

Thanks!

PS it's really hard to actually post in this forum >:(