WICID call back - server registration


When using the API “wiced_http_server_register_callbacks”, there is an optional parameter to register a callback function which is called when a packet is received by the server. Are there any constrains for the execution time within that call back? Based on the packet timeout, may need to construct the html page to return (which involves communicating with another device over serial connection). Dont have all the timing parameters yet, but in general, can/should processing be done in the callback or should be delegated to another thread?