Listen, preload puts it into ram but you have to assume the player has 4gig, that's a LOT. 8 is the norm these days. Doing the heavy lifting at startup means no glacial memory fetches when the game is running and inevitable stalls.
The rule of thumb in optimisation is this: if it's not a problem yet then it's not a problem so forget it. Unless you have some crazy requirement that you know will grind everything to a halt, just ignore it and if it becomes a problem, then deal with it, do load threading or whatever.
Remember, your productivity is more valuable than your machine's. If dev takes so long that you never make the game then you don't have a optimisation issue; because you have nothing.