The site has reached the limit of the home directory and is slow to load. How do I increase the limit?
Assuming you have WHM root access on the server, You can create /home2 directory. When multiple home directories exist on a server, the account creation task in WHM will automatically assign the user to the path with more available space. After creation, the account can be moved to the desired home directory using the WHM feature "WHM >> Home >> Account Functions >> Rearrange an Account": https://documentation.cpanel.net/display/78Docs/Rearrange+an+Account
You can either attach one more drive to the server and follow advise given be hostechsupport to host new accounts on new drive however if you do not want to buy one more drive, you can remove unwanted accounts from your server. You can also check whether you have unwanted backup files stored in your home directory. Those can be removed. If you have enabled cPanel backups for user, you can use find command to scan through all your accounts and make sure to remove large number of cPanel backups.
Do you have root access? Check which users are using the disk space, delete files that are not needed, these should help.
Talk to your host and ask for more space and see what they offer. And make sure that users limit is working or they be all unlimited space.