Thanks to ali1234 this was tracked down to font-config rebuilding the cache at the start of the live session. Something changed in the way fontconfig validates the caches and so they are being rebuilt at the start of the live session.
This bug is likely related: https://bugs.freedesktop.org/show_bug.cgi?id=103652 ** Also affects: fontconfig (Ubuntu) Importance: Undecided Status: New ** Also affects: fontconfig (Ubuntu Bionic) Importance: Undecided Status: New ** Also affects: casper (Ubuntu Bionic) Importance: Undecided Assignee: Canonical Desktop Team (canonical-desktop-team) Status: New ** No longer affects: casper (Ubuntu) ** No longer affects: casper (Ubuntu Bionic) ** Changed in: fontconfig (Ubuntu Bionic) Assignee: (unassigned) => Canonical Desktop Team (canonical-desktop-team) ** Bug watch added: freedesktop.org Bugzilla #103652 https://bugs.freedesktop.org/show_bug.cgi?id=103652 ** Changed in: fontconfig (Ubuntu Bionic) Assignee: Canonical Desktop Team (canonical-desktop-team) => Iain Lane (laney) -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to fontconfig in Ubuntu. https://bugs.launchpad.net/bugs/1749546 Title: Booting a live session is slow Status in fontconfig package in Ubuntu: In Progress Status in fontconfig source package in Bionic: In Progress Bug description: On Bionic Ubuntu Desktop booting to a live session is slow. It takes roughly 2 min on my test machine to boot from the "Try or Install Ubuntu" boot menu to a usable session. It also takes 1 minute to logout and 17s to log in again. ProblemType: Bug DistroRelease: Ubuntu 18.04 Package: casper 1.388 ProcVersionSignature: Ubuntu 4.13.0-32.35-generic 4.13.13 Uname: Linux 4.13.0-32-generic x86_64 ApportVersion: 2.20.8-0ubuntu10 Architecture: amd64 CasperVersion: 1.388 CurrentDesktop: ubuntu:GNOME Date: Wed Feb 14 16:43:19 2018 LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180214) ProcEnviron: TERM=xterm-256color PATH=(custom, no user) XDG_RUNTIME_DIR=<set> LANG=C.UTF-8 SHELL=/bin/bash SourcePackage: casper UpgradeStatus: No upgrade log present (probably fresh install) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/fontconfig/+bug/1749546/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : firstname.lastname@example.org Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp