I tested if increasing the buffer made a difference: $ stdbuf -i8191 -o8191 paste -d, /dev/null

Top still shows that it is the outer paste that is the bottleneck. # Paste 11% at a time in parallel, and finally paste these $ paste -d, file* | pv | head -c 1G >/dev/null That improves throughput slightly, but it is still a magnitude slower than pure pv: # Baseline Thanasisp suggests running jobs in parallel. The input files are UTF8 and so big they do not fit in RAM, so reading everything into memory is not an option. Is there another tool that does the same, but which is faster? Maybe by reading 4k-64k blocks at a time? Maybe by using vector instructions for finding the newline in parallel instead of looking at a single byte at a time? Maybe using awk or similar? Looking at the source code it is probably because it uses getc: while (chr != EOF) Paste is using 100% CPU according to top, so it is not limited by, say, a slow disk. Paste is a brilliant tool, but it is dead slow: I get around 50 MB/s on my server when running: paste -d, file1 file2.

Quick test paste