Cry havoc! And let slip the pigs of war!
Precisely this.Which is funny, because I don't run mods because I'm too lazy! I run the same campaign world, reusing locales and old lore modified by previous campaigns because it's easier. I can think up different overall factions and goals while I'm taking a shower, jot down some notes and spend a few minutes with a random name/NPC generator and I'm most of the way there. I never get too worried about detailed maps because exploration is mostly TotM and the most work is flipping through the MM (or DDB, now) to find the appropriate monsters that would be a challenge.
Meanwhile running a mod I have to actually understand the whole thing, write notes, remember where references are. It's far, far more work to run someone else's stuff than my own. There are only a handful of plots for campaign arcs. Off the top of my head there's rescue, escort, loot, defend, investigation/mystery, the hunt. As a DM I just change the set dressing so I'm not really reinventing much of anything.
Since I have a campaign world of my design with factions and religions and such I can have a donjon dungeon produced and quickly devise a backstory.
When things happen in the adventure I already understand why and can improvise without it feeling like mere whim at the moment.
Some things take longer—-but I really struggle with the ‘adventure paths.’
The difference is “knowing” vs. learning and remembering.