When first talking to TT, Klein says if you finsih the quest quickly, he will give you an old world gift. What is it, and how fast do i have to finish? Also which quest line? If its the main quests, can I not do the others after completing main quest?

Thanks

He doesn't have a gift, what he says is he'll show you Old World thanks, or something like that.

If I had to guess, it's the developers' way of making fun of how people will rush through games and DLCs and such and whine about how it was too short, or there wasn't enough stuff to reward them.

Klein's lying to you; once completed, he informs you that your usefulness has expired, he'll try to dispose of you.