As the topic says , my quest doesn't start until later. I talk to the NPC and nothing happens. After 20 seconds i come back to talk again and now he's more cooperative.

did you create a seq-file for your mod?

Even with a SEQ file I think it can still be buggy sometimes, most times I've hit issues like yours is when starting a new game with the default PC and COC from the main menu, then it's been a case of needing a trip through a load door or a save and reload before the dialogue runs properly. In general gameplay with your mod installed most likely the player is already going to have saved and loaded many times before they hit your quest.

y, i have seq files!

How are you accessing your quest? Coc from main menu? Load a save then run over? Load save then fast travel? Load save then coc? Etc.

that's why i don't use that method at all anymore.

instead, i use a script on a persistent ref that starts my quest OnInit.

works as reliable as can be, i still wouldn't necessarily recommend doing it like this though, if it was ideal, they'd have done it in vanilla game

I test my quest via a saved game where my player is near the cell that NPC is located. It's a free mod save (i mean, i saved with my mod uninstalled).

I went there via load save then coc.

Ok, good. That's the best way to test new quest content. On that note, things do need to get booted up, the game expects that you have the quest in your game from the same you start a new game, so it assumes it has time to set everything up. Try doing a quick 1-2 hour wait ingame when you load your save. Then run/coc over there and see if it works instantly.

Yeah, works fine now. Then, is it normal?

Yeah that's normal, I could be tempted to say SNAFU but that might be a little harsh on the game since it is trying to do a lot of things.

Yes it's normal. The game isn't designed for you to load up brand new content on a save that's never seen it before and expect it to work instantly. Quest content has to 'boot up'.

Static things like new dungeons/meshes/etc are instantaneous as they're just loaded into the game with the save. They don't do anything at runtime, and aren't managed by any overhead systems.

Ok! thx for your time and patience.