If you are using in WiFi repeater mode on a single band router like the AR300M (2.4Ghz only ) then normally you would expect to only get around half the bandwidth obtainable on the original router. You have to remember you are now using the one band to not only connect from the AR300M to the original router and also to connect your WiFi device to the AR300m so effectively halving your bandwidth. Add on top of that potentail 2.4Ghz interference from other 2.4Ghz WiFi routers potentially reduces it further.
What Alzhao was indicating was that if you have a dual band router then you repeat on one band using it’s “full” bandwidth and then use the other band’s “full bandwidth” for your WiFi LAN network.