I would like to know if scenarios like FfH normal games are advised to be played at normal speed due to balance issues (i'm particularly referring for example to XP which at Marathon speed become unbalanced)?

In general would you advise me to play scenarios at Marathon so that i get the same balance and same feeling if those scenarios were played at normal speed?

I usually play them on quick speed and warlord difficulty. I want to immerse myself in the story not go grey over a protracted slogged battle.

I think the scenarios were devised for normal speed. Only HE who is above all others could be more precise.