Improve Growing Cell Array to Cluster Points

Hi! I would like to know how to efficiently run this part of my progam. Basically for a background of what I aim to do, I want to cluster points. There are around 10,000 points. These points have computed "forces" between them that is stored in matrix F. So the force between point 1 and point 2 is F(1,2). I would then like to "cluster points" with sufficient F acting on them (Force setting/threshold), that is 2 points with sufficient F between them belong to the same cluster. I have a code as seen below. A cell array CLUSTER was made to contain the cluster assignments. So CLUSTER{i} is the cell containing the ith cluster with clustered points in it. However, for some F settings, the implementation takes forever. I have read about preallocation and parfor (parfor can't be done since there is dependency in iterations). But does preallocation of cell arrays mean that individual cells are not preallocated with memory? Is there any other way around this? Profiling tells me that ismember has the biggest share in computing. I hope to improve the code with your suggestions. Thanks a lot!

    CLUSTER = {};

    for fi = 1:srow
        for fj = 1:scol
            if fj > fi % to eliminate redundancy, diagonal mirror elements of F !check on this
                if F(fi,fj) >= 2000 % Force setting

                    if( (~ismember(1,cellfun(@(x)ismember(fi,x),CLUSTER))) && (~ismember(1,cellfun(@(x)ismember(fj,x),CLUSTER))) ) % fi & fj are not in CLUSTER
                        CLUSTER{end+1} = [fi fj];
                        end
                    %%if( (ismember(1,cellfun(@(x)ismember(fi,x),CLUSTER))) && (ismember(1,cellfun(@(x)ismember(fj,x),CLUSTER))) ) % fi & fj are in CLUSTER   
                        %do nothing since lfi and fj are in CLUSTER    
                    %%end    
                    if( (ismember(1,cellfun(@(x)ismember(fi,x),CLUSTER))) && (~ismember(1,cellfun(@(x)ismember(fj,x),CLUSTER))) ) % fi in CLUSTER, fj not in CLUSTER
                        c = find(cellfun(@(x)ismember(fi,x),CLUSTER));
                        CLUSTER{c} = [CLUSTER{c} fj];
                    end        
                    if( (~ismember(1,cellfun(@(x)ismember(fi,x),CLUSTER))) && (ismember(1,cellfun(@(x)ismember(fj,x),CLUSTER))) ) % fi not in CLUSTER, fj in CLUSTER
                        c = find(cellfun(@(x)ismember(fj,x),CLUSTER));
                        CLUSTER{c} = [CLUSTER{c} fi];
                    end
                end
            end
        end
    end