Building paperback is slow

[Sjors]

./make_book.sh --paperback takes really long for me.

On a 2019 Macbook Pro (2,3 GHz 8-Core Intel Core i9):

165,75s user 21,38s system 99% cpu 3:07,79 total

On a 2017 iMac (3,4 GHz Quad-Core Intel Core i5):

207,83s user 27,46s system 55% cpu 7:06,18 total