I'm not super elegant at this but would think you would want to do it in one command rather than adding a column, then adding a column, etc., as a loop. I have been working with huge datasets and it can crash matlab on my older work machine when I make Matlab do extra work.
Maybe rather than horizontally concatenating in a loop, you could create a loop that writes the list for concatenation, then just insert the list?
I'm just sort of pseudo coding this out...
% initilize
x = total number columns
counter= 1
hugeFlippingString = empty string
% run loop
Loop until counter = total number of columns
append text to hugeFlippingString: "columnOrFileName", counter, ","
counter increment by 1
End loop
%Concatenate
giantMatrix = horzcat(hugeFlippingString)
% which actually does this: giantMatrix = horzcat(col1,col2, ... col100)
