If you did not have full HP after a rest - doesnt it just slow down the game?
It depends.
In pre-4e, it did. It took forever to naturally heal so a party that adventured more than once a week/month needed a healer caster of a steady supply of healing potion.
That's why I suggest rest to just restore HD.
Spending all your HD restores about 40-60% of your HP.
From the Playtest at level 1, that is:
40-45% for the fighter
25-31% for the rogue and wizard
29-35% for both clerics
So it takes 2-3 days to go from 1 to full HP. 1-2 days if you have someone with the Healer theme.
Then after 2-3 days, everyone is healed to Max HP. One more days restores everyone's HD if they want that back to