So do you want to be 'fast' (as per your title) or 'lazy'? Anyway, you can be slightly lazy by doing
for i in rooms_dic:
var room = rooms_dic[i]
(i.e. don't need ".keys()").
You can get an array of the values or keys by doing
var dict_key_array = Array(rooms_dic.keys())
var dict_val_array = Array(rooms_dic.values())
and then iterate the array. What more did you want?