I am going to go on a limb here and believe you mean the wifi speed ? as in AC1750 or AC1200 or AC2600 ?
The answer to that is assuming your router is AC1750 and you get a repeater that is AC750 for example, your router will adjust to match that AC750 speed, but if your repeater is AC1750 and your router is N900 for example, then the repeater will not even use AC1750, because the router isn't able to use it at all. So best try to match router abilities with your repeater abilities the best you can
what is the make and model your Router