I mean, why does it even suffer from script lag after you install 4 or 5 mods ? In Oblivion or Morrowing, one could run many different scripted mods and they would work without any lag. I have never seen anyone complain about script lag in the older TES games, so why is Papyrus so different ?

Because its a threaded language and more care is needed in coding?

I assume it was a design decision. For high-level scripting ("run package on actor", "unlock door", "add reward items" etc.) latency is usually not a critical property.