Welcome to the forum, Emms.
Are you opposed to using Mods (modifications)? If so, try
Overwatch by twallan. I also use
Delphy's Dashboard but that might not be of much use to you (it mainly organizes CC, but can also help clear extra caches).
How long have you had your computer, and is it a Desktop or Laptop? If you've had it for a while, try opening the case and removing the dust. Or ask a tech-oriented person to do it for you.
Do you have a lot of processes (tasks/applications) running at the same time as Sims? If so, try stopping some of those through closing them, or using Task Manager.
Can you tell us your Operating System and RAM amount (for example: Windows Vista, 4GB RAM)? That could tell us whether you actually need more RAM.
Sims can be a bit RAM-hogging, so you may find that you need to increase it.