Just to add to Steve's comment. 802.11n clients tend to stick to 802.11n access points so they prefer to roam to another 802.11.n AP. You should look at the client load on the 1131's compared to the other access points. If you see more clients associated to the 802.11n AP's, that can be why. They might be connecting at a lower rate due to the client sticking to an 802.11n access point than roaming to an 1131.
You need to diagnose what the slowness is also, is it when they are connect to AP x, what are they connected at, do you see anything on the WLC logs, is it reproducible?
1. What kind of wireless clients are you using? 2. What kind of wireless clients are complaining of slowness? 3. Does this issue happen all throughout the day or during certain times? 4. What is the signal strenth from the client(s) when slowness occurs? 5. What protocol does this happen? 802.11 a/b/g/n?