Trying out HEAD (specifically, ghc-6.13.20100831-src.tar.bz2 built with 6.12.3) investigating an issue with the text package, I found that I/O of ByteStrings has become significantly slower (on my machine at least: $ uname -a Linux linux-mkk1 2.6.27.48-0.2-pae #1 SMP 2010-07-29 20:06:52 +0200 i686 i686 i386 GNU/Linux Pentium 4, 3.06GHz). Timings for reading and outputting a 74.3MB file: cat: $ time cat bigfile > /dev/null 0.00user 0.04system 0:00.06elapsed 83%CPU ghc-6.12.3: $ time ./nbench lazyBSNull bigfile a b > /dev/null 0.01user 0.09system 0:00.10elapsed 100%CPU ghc-6.13.20100831: $ time ./hdbench lazyBSNull bigfile a b > /dev/null 0.07user 0.10system 0:00.18elapsed 96%CPU In addition to the slowdown, the allocation behaviour has become quite bad: ghc-6.12.3: 89,330,672 bytes allocated in the heap 15,092 bytes copied during GC 35,980 bytes maximum residency (1 sample(s)) 29,556 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation) ghc-6.13.20100831: 475,305,720 bytes allocated in the heap 89,272 bytes copied during GC 68,860 bytes maximum residency (1 sample(s)) 29,444 bytes maximum slop 2 MB total memory in use (0 MB lost due to fragmentation)