Low internet speed on PLW1000v2 Extender

what are you doing wrong? Nothing. It can be mulitple issues. 

Many people see outlets next to each other or just 1 room over and think "they're close to each other, they should work great". When many times, those outlets have quite a bit of wiring between them because they might be on different circuit breakers. so they might be within 10ft of each other but have 100+ft of wiring in between them. When they're on the same circuit, their performance is quite amazing but add a circuit inbetween them, and performance drops off quite a bit. 

also, many things can interfere with powerline. Such as: 

high draw appliances

Old/bad/poorly done wiring

gfci outlets

arc fault outlets 

surge protectors

electronics going bad

cheap cell phone chargers

distance

Poorly made electronics

This can all inject interference in the powerline system.

There's also a misunderstanding in performance. Powerline numbers are over inflated more than wireless and thats already pretty bad. 

I'd take a read through actual testing on the PL1200. Its a step up from yours but should give you some actual performance data to go off that is pretty reliable. 

https://www.smallnetbuilder.com/lanwan/lanwan-reviews/32717-netgear-pl1200-powerline-1200-reviewed