Your home folder is, /home/user, no surprise.
By normal means you won't be able to access this folder on other machines except you use SSH or SFTP.
Locally (on the phone) itself you'll be able to access it via the terminal, so to put a rundown of what you need to do:
Figure out config folders of
apps you don't really use anymore. Highlighted that in red and in bold, because it's important so that you do get to know what's inside the /home/user/ folder. One mistake for one app and your settings for that one app all goes ka-pish. Those folders are usually hidden and they usually have a period ( . ) prefix before the folder name. Identify those folders and delete them.
By the way do take note some folders especially /home/user/MyDocs. This folder is where your N900's internal storage is, and the one you usually access when you plug in your N900 via USB Mass Storage mode on PC.
Also one of the possible causes of lag (may vary from person to person) is because of your uptime, where heavy swapping occurs between the phone's internal RAM and the swap memory of the phone. Usually what I'd do is a reboot when the lag gets too unbearable (I've missed several calls thanks to it!).
One more thing: I recommend issuing the command (as root):
CODE
apt-get clean
in order to clean the files your app installers (Application Manager, Faster Application Managers, apt-get, what have you) have downloaded.
Thanks for the suggestion, I will try the apt-get clean command first, I rebooted my phone for few times, everything is working normal, just the overused HOME kinds of worried only. I don't heavy tinker my phone, I suspect is all those cache file never cleaned :-)