Part of this is fixed in c9f2977 (on main, in the next release):

• A card's change list now counts only its own work: it is measured from where the card left the branch it is built on (the default branch, or a long-lived branch like yours), not from the default branch.
• Change lists no longer block the server: they are read in the background, for all cards at once. Changed line ranges are read for the first 200 files of a card.

Demo: https://demos.obeya.si/github-issue-2-537d52/

Still open, so I'm leaving the issue open:

• git worktree add and the clone's fetch on a card's start still block the server. That's next. Until then, clones instead of worktrees avoid the slow worktree add.
• Suggestion 3 (navigation while disconnected) isn't done: one server serves every canvas, so when it doesn't answer, no other canvas would either.