Hello Patrick,
It could be a couple reasons but were you using 20mhz band or 40mhz band? Also there could just be a lot of wireless interfance going on the 2.4ghz that is slowing your speeds down. Usually no one is using 5ghz so there may not be anyone on those channels around your area.
Do you know what your wireless speeds were on your network card between 2.4ghz to 5ghz? If you can stick to 5ghz though that is the way to go but the problem usually with this is that fact that there isn't a lot of wireless device that can connect to 5ghz.
Hope that helps out.
Thanks,
Clayton Sill