The core reason for the issue may be, the iPad is using STATIC IP assignments in 'Settings > WiFi'. You may need to check the IP assignment settings. IAlso, you MUST ensure that you also enter a DNS IP address in the DNS field and for good measure enter the same in the Router field if not already indicated. Apple's walled garden approach to networking contributes to this problem with iOS thinking that it has no network routing path, and as such limits or prevents traffic outside of port 80.
For further similar related information, please see our Knowledge Base here: Sluggish, slow feedback on Apple device