What make and model wireless extender? Do need to be certain about the device in question. Terminology is sometimes mixed around.... Access point perhaps?
Reference:
https://www.blacknovadesigns.co.uk/blog/wireless-access-points-vs-wireless-repeaters
You can google and find more explanations and details.
Wireless is inherently slower than wired.
And a network will only perform at the speed of the slowest link.
As for the 2/4 bars (signal strength) they are often just a rough indicator that does not relate to speed per se.
However, weak signal strengths may result in corrupted/interrupted transmissions and continual packet resends. Thus causing performance drops.
That all said: consider using power line adapters.