To clarify it for all, NEVER EVER USE A WIFI EXTENDER.
You place a WiFi extender at a place, where the wifi is already bad. After adding this extender it looks better. But what happens is, this extender us a bad signal with bad performance and create a new signal. This will give you less performance as without extender.
As all other write, use a Bridge or Mesh. But same for this, if the Mesh client have a bad signal, you wireless client will habe a bad signal. Although it shows a good signal, but this good signal is from Client to AP, from AP to AP to LAN you have a bad connection.