There are some global variables GEngine like GAverageFPS and GAverageMS that can be very useful.

They are declared in GEngine, but to access them from your own code you will need to use the extern keyword:

#include "Engine.h"

void UBUIUWMyStatWidget::ShowFPS()
{
	extern ENGINE_API float GAverageFPS;
	extern ENGINE_API float GAverageMS;

	FPSLabel->SetText( FText::AsNumber( GAverageFPS ) );
	MSLabel->SetText( FText::AsNumber( GAverageMS ) );
}

That’s it!