Thanks boneyard for your response.
below is the list what i did to spot the actual issue..
i pinged the default gateway and vc [ from a device which was connected wirelessly] and shocked the avg response is around 150ms
also i pinged the gateway and vc from a wired connected device and the avg response was only 2ms.
they have MPLS [100mbps, so no question about wan side issue] and all was working fine on wired.
Before that the never faced that issue. and there wireless was extremly fast. [IAP 225]
So i confirmed that the issue is on wireless, after that i found that they was suffering from interference issue.
So today i went there and disable some of the unused SSID, increased the immunity level, and made that wide channel bands as none.
Now the things are looking good.