Speed questions.

First - because of the laws of physics, higher frequencies suffer more from attenuation, so, at any given distance from your primary WiFi source (whether it be an access point or a router), the 2.4 GHz signal is going to be stronger than the 5GHz one - this can lead to lower connection speeds. Second - look closely at the spec - do you see the words "up to"? No manufacturer guarantees a given connection speed - the actual connection speeds will vary with the environment - physical building construction, distances between the devices, interference from other WiFi networks or electronic devices, all of these have an impact. The mere fact that you have purchased an extender is indicative that you are aware of the connection issues, the extender will alleviate but not eliminate these issues.