Are there any time limits?
Are days important or just only for journal?
I am now at Reapers Coast, and after ritual, I was informed that finding masters of source is my main priority and nothing else matters, and I must do it as fast as possible before others will reach Well Of Ascension.
Is that true in practice or can I back to my slow play with explore-and-touch-everything and doing smaller side quests?

I don't believe so. Days don't really mean anything except a rough measure of in-game hours.

Time affects very little in the game.

Only a few NPCS will move around based on their quest path (like Tarquin visiting the cemetary and if he stays along enough, he will fight a nearby grave monk before returning to the ship) and take time to reach from one point to another.

Other than that, there's the merchant restock every 1 hour.

You can progress at your own pace. Let's be honest, every RPG is like this. "Make haste, hero! Fate of the world depends on you!" -- "Ok, old man. I shall vanquish the evil and-... Ooh a flower. Ooh a sidequest. Ooh a farm I can build. This dragon I've literally stopped one step from getting a scripted encounter with ain't going anywhere."

about gareth


this is also not really time related but if you rescue gareth during act 1 before finding the seekers' camp and healing the injured warriors, upon reaching the camp you'll find said warriors all dead (if you find the camp first, you can heal the warriors for a couple of rewards, but i believe these can be looted off their bodies). more to do with the way quests are completed than time

other than that, there's nothing i know of

If you play Ifan, the elf vs thug quest should progress like EA and they should fight once you reach town for the conversation trigger to happen and enough time pass for them to finish talking.

and there' Doctor Leste and the injured guy. Leave long enough and he dies.

The time only start ticking if you trigger the NPC flags first.