Arduino to unity3d & Ipad

is this a wifi shield related problem?

Maybe. Or a coding issue. My bet would be a coding issue.

can we remove the lag using some wifi firmware?

Doesn't seem likely.

should i simply use the redpqrk serial cable?

Does that work for your application?

should i use a different wifi shield? how about the wifly shield?

What about it? What wifi shield are you using now?