Wireless range extenders aren’t generally known for their performance, particularly when you compare their theoretical or advertised data rates with what we see in the real world. Frankly, most folks consider range extenders when they're faced with a shortage of wired ports or some corner of the house that can't maintain a reliable wireless connection. Performance isn't necessarily the priority in those cases. But regardless of your application, throughput is still going to be a consideration, even if you know you'll never see the speeds printed on the box.
Wireless networks are notoriously susceptible to performance degradation due to signal loss and interference. The results you measure from your wireless components have as much to do with their placement as the distance between them. Wireless range extenders are doubly susceptible to signal loss since they function as both a wireless client and access point, with each segment typically running on non-overlapping channels so they don’t interfere with each other (but increasing the likelihood of interference from another source). Understanding these limitations and tempering expectations appropriately is necessary given the nature of wireless networking in general and the nuances introduced by range extenders.