AC1200 WiFi range extender installed, thru WPS, works but only 25 % of WiFi speed on terras.

A couple things. 

1. 2.4ghz is a slow network. You didn't put if you're testing of the 2.4ghz or the 5ghz. 

2. single and dual band extenders cut throughput 50% of what they're receiving. This has 2 impacts. 

• first is thoroughput reduction of 50%. They use the same chip to go router----extender and then extender---device. and they can't do both at once. So their speed is 50% 
• That speed is 50% of what they receive. Say if you're starting with your router putting out 100mbps over 5ghz. 5ghz drops off quicker with distance and obstruction for speeds. So at the spot where you put the extender, the router is only able to transmit at 60mbps. The extender is only going to be able to put out 30mbps. So it cuts 50% off of what its receives

3. You didn't put what router you have. A crappy router is going to have speed drop offs quicker than a decent quality router. 

4. Distance. If its to far away, its not getting that great of a signal from the router and is already starting at a low point

5. Obstruction. 2.4ghz can broadcast further and through more materials but its slow. 5ghz gets blocked easier and doesn't broadcast as far but its a much faster network. 

6. Actual values. You didn't put what actual values you got to determine if your speeds are within what we'd expect for that throughput losses

7. home size and what its made of. Certain materials block wifi better than others. Interior walls of plaster lathe are going to block wifi quite well. As well as size is important.

It could be a combination of the factors. Having a base model router, having the extender to far away so its not getting a decent signal. To large of a home, home made with materials that block wifi, etc. You gave us very minimal info to go off so its tough to tell you if there's actually a problem or if this is an expectation of your setup.