HI,
The given information is not enough to diagnose the issue. but let me give you clarity on both methods.
Whe DHCP is selected as VC assigned, client traffic will be NATed with the VC IP and go out of the IAP. and if we choose network assigned, client traffic will go out of the VC with out NATing hence client will be treated similarly as other wired Clients.
As a work around, check your firewall whether any bandwidth restrictions are configured for the IAP subnet.
Please feel free for any further help on this.