I suggest you to tell TestComplete to go and find the objects with in the specific browser you are going to run.
Set UsernameTextbox = Sys.Process("iexplorer").FindChild(Array("ObjectType","ObjectLabel","VisibleOnScreen"),Array("Textbox","Username","True"),10,True)
or
Set UsernameTextbox = Sys.Process("chrome").FindChild(Array("ObjectType","ObjectLabel","VisibleOnScreen"),Array("Textbox","Username","True"),10,True)
If you ask TC to find blindly on the whole system, then obviuosly it would take lot of time. Have some Project Variable for whic browser to run and select the browser while start running.