The campaign is a tutorial with a long runtime
The campaign teaches the systems one at a time. It is also the only part of Path of Exile 2 where you are guaranteed to be underlevelled for something.
Read it that way. The goal is not speed, it is arriving at the endgame with a character that understands its own build.
Pacing: stay near the area level
Enemy scaling is not gentle. If you find a pack taking far longer than the last one, you are behind.
The fix is not a new build. Clear an earlier area you skipped, or run a side area for levels, then come back.
Gear during the campaign
Do not farm. Campaign items are replaced within a few levels, so anything you invest in is wasted.
The exception is a socketed item with the right number of links. That one is worth keeping and worth putting supports into, because it carries your skill for several levels.
Bosses that stop underlevelled characters
Every act ends with a boss, and each one is a gear and resistance check rather than a skill check.
Before a boss, do three things: cap your resistances, fill your flasks, and make sure your main skill has at least one support gem. That covers the majority of deaths.
Do not skip the trial content
The Trial of the Sekhemas is available during the campaign and it feeds your ascendancy. Running it early is the difference between a build that feels finished and one that feels unfinished for another twenty levels.
The ascendancy guide has the exact requirements per step.
Skills to unlock early
Two things make the campaign easier than any gear upgrade.
The first is a movement skill. Anything that closes distance or escapes a pack reduces deaths more than any stat.
The second is a second element of damage. Bosses resist, and a character with only one damage type stalls on the ones that resist it.
What to carry into endgame
By the end of the campaign you should have: a main skill with supports, capped resistances, a movement skill, and a passive tree that points at one damage type.
Everything else, you can fix later.