If it's just performance, cd ~/Documents && mkdir old && mv ./* old/ (or today's date instead of old). I actually have that layout on one PC.

If real organization is needed, seems like that'd be easier in hindsight than having foresight

So then you have one intentionally slow directory ("old/" in this case) and one fast directory?

Personally I'd categorize stuff, but you do you, there really isn't any wrong way to do it, if it works it works :)