ismember runs too slow (others have asked similar questions)

I am trying to use ismember in a long for loop to identify indices shared between arrays. This is pretty similar to a question that was asked here. Hopefully, an answer can be found for my question.

Below is my script that is taking far too long to run. I've included a sample of my workspace with the important variables.

 Lcu =length(count_uni);
 for i=1:Lcu
    count = count_uni(i);

    k = find(count_all==count);
    % count = 1: 1 unique values
    % count = 2: 2 unique values
    % etc.

    PosAllk = PosAllrep(k,:);
    A_allk = A_all(k);  PVTr_allk = PVTr_all(k);
    count_allk = count_all(k);

    if count>1
      LPAk = length(PosAllk);
      for j=1:LPAk
         b = ismember(PosAllrep,PosAllk(j,:),'rows');

         Aavg = mean(A_all(b));  Pavg = mean(PVTr_all(b));

         g = ismember(PosAlluni,PosAllk(j,:),'rows');

         TimeAvg = [TimeAvg;[PosAlluni(g,:),Aavg,Pavg,count]];
      end
    else
        TimeAvg = [PosAllk,A_allk,PVTr_allk,count_allk];
    end
  end