and key point:
"Previous benchmarks we have run (where we rebuilt the entire archive for x86-64-v3 57) show that most packages show a slight (around 1%) performance improvement and some packages, mostly those that are somewhat numerical in nature, improve more than that."
Around 2000 i found that if you have document with header/footer with auto page numbers and merge it with few pages from other document also with page numbers you get huge mess in numbering.