Why do extenders sometimes cut my speed in half?

Specialist subreddit threads repeatedly flag this: a wireless-backhaul extender has to receive on one radio and re-transmit on another, which can halve throughput if both legs share a band. The fixes mainstream reviewers recommend are (1) place the extender halfway between the router and the dead zone, not inside the dead zone, (2) use a model that supports dedicated 5 GHz backhaul or 160 MHz channels, and (3) wire the extender back to the router over Ethernet if at all possible.