The warning might not be the only reason for your compilation to become slow. The problem might be somewhere else also.

Qt Code: 
  1. mainform.ui.h(476) : warning C4100: 'action' : unreferenced formal parameter
  2. mainform.ui.h(996) : warning C4100: 'val' : unreferenced formal parameter
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These warnings mean that you are passing the parameters action and val is some function but you are not using them anywhere in the function.