I’ve said this before, but it bears repeating because people on the internet… a range extender is only as good as the signal it gets. If it’s getting a terrible signal from the base unit, it will give you a very strong signal but very slow internet because it’s getting slow internet.
Most internet comes into the house near an exterior wall. For the best signal if you’ve got an ISP’s cable/fiber modem with Wi-Fi it should be centrally located in the middle of all things that use Wi-Fi, but that’s usually not an easy option. Line of sight with minimal things in the way blocking the signal.
If you get a range extender it needs to be placed somewhere where the signal is good in order to get a new signal somewhere where the signal is bad. If you can’t centrally locate your cable/fiber wi-fi modem, then that’s where you want to put a repeater/range extender.
And that’s that.