Yes I have the QL420 + printers working with 5508 WLC and 3502E CAPWAP APs and PEAP
Fotis - You will most likely find the reason for the slow ping resonce is down to the setting for "Power Mode". You likely have it set to "best". This setting controls how long the device "sleeps" before it awakens and downloads queued traffic from and AP. Setting it to "off" will put the device in to CAM (Constantly Awake Mode). This means that the device never switches its radio card off and never allows traffic to be queued on an AP. However this will mean that the drain on the devices battery will be much greater, I believe there is a slidding scale of settings for this device that go in order of highest battery drain as follows:
Off will give you the best performance with maximum battery drain. play with the settings and see which gives best performance/batery drain balance.