I found a system, that can do it without displaying   
   it, in around 8.5 secs. Thats kind of fast, isn't it?   
      
      Welcome to SWI-Prolog (threaded, 64 bits, version 7.7.15)   
      SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.   
      
      ?- [user].   
      pow(_,0,1) :- !.   
      pow(X,1,X) :- !.   
      pow(X,N,Y) :-   
           M is N//2, pow(X,M,A),   
           J is N mod 2, pow(X,J,B),   
           Y is (A*A)*B.   
      ^D   
      
      ?- time((pow(9,9,X), pow(9,X,_))).   
      % 159 inferences, 7.500 CPU in 8.514 seconds (88% CPU, 21 Lips)   
      
   See also:   
      
   Whats the fastest way to compute 9^(9^9)   
   https://math.stackexchange.com/q/2808923/4414   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)