Looking at the charts above (Plotting process measurements - #3 by gladanimal, Plotting process measurements - #23 by gladanimal) I found using default buffer size (3389MiB) affects to (especially) composing tables 4 and 5 at first phase and thus increases temporary storage I/O. So using 5000MiB would be optimal for performance on k=32 and u=128. Higher buffer size would not be utilized!