Wifi extender can only transmit max speed that it receives. If you place it somewhere with weak signal, it can only extend that weak signal. So change and play around with the extender placement if possible. Easier if you install some wifi analyzer app on your phone and look at the wifi signal strength at the place you want.

If using android, I suggest this app for checking wifi signal:
https://play.google.com/store/apps/details?...em.wifianalyzer

No ads, easy to use, it also show details about other wifi signal in the area. You can then adjust wifi settings on your router to avoid congested channels.