we had a similar issue at one of our customers around two years back with iOS devices. The download speed was ok, but the upload speed was bad (compared to other devices). It turned out to be a driver incompatibilty between the APs (using AP8432, AP7532 and AP7522) and the iOS devices.
Solution for us was to upgrade the firmware of the APs to 22.214.171.124, because there was a new broadcom driver included. So you might test to upgrade one of the APs to 5.9.2 (or higher) and see if it gets better.