Brains are resource-intensive only if you are in a massive hurry. Starfish do fine, entirely without a structure you would call a brain. Packing it all into one noggin is risky, for no benefit. If you're not in a hurry, you don't need nearly as much of it, because it doesn't have to do everything at the same time.

There is no natural speed that life has to operate at. Any aliens we encounter are much more likely to be very, very slow, because it is they who can stand the vast chill void between stars. If they visit, and you dare to chop one down, you wouldn't find anything recognizable as brain inside.